Fix Windows test compatibility for ProcessBuilder usage

On Windows, Java's ProcessBuilder cannot directly run shell wrappers
like `npx` (installed as npx.cmd) or Unix commands like `cat`. Tests
that used these commands failed with "Cannot run program" errors.
Additionally, Unix-style paths like "/nonexistent/copilot" are not
absolute on Windows, causing assertThrows(IOException) tests to pass
unexpectedly when CliServerManager wrapped them with "cmd /c".

Changes:
- CapiProxy: use "cmd /c npx" on Windows to launch the test harness
- CliServerManagerTest: replace "cat" with cross-platform dummy process;
  use a platform-appropriate nonexistent absolute path so IOException
  is thrown on all platforms
- JsonRpcClientTest: replace "cat" with cross-platform dummy process

All changes use runtime os.name detection and preserve existing
behavior on Linux and macOS. Full test suite passes on all platforms
(556 tests, 0 failures, 0 errors).

Author: edburns

src/test/java/com/github/copilot/sdk/CapiProxy.java

@@ -89,7 +89,11 @@ public String start() throws IOException, InterruptedException {
         }
 
         // Start the harness server using npx tsx
-        var pb = new ProcessBuilder("npx", "tsx", "server.ts");
+        // On Windows, npx is installed as npx.cmd which requires cmd /c to launch
+        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
+        var pb = isWindows
+                ? new ProcessBuilder("cmd", "/c", "npx", "tsx", "server.ts")
+                : new ProcessBuilder("npx", "tsx", "server.ts");
         pb.directory(harnessDir.toFile());
         pb.redirectErrorStream(false);
 

src/test/java/com/github/copilot/sdk/CliServerManagerTest.java

@@ -75,8 +75,11 @@ void connectToServerStdioMode() throws Exception {
         var options = new CopilotClientOptions();
         var manager = new CliServerManager(options);
 
-        // Create a dummy process for stdio mode
-        Process process = new ProcessBuilder("cat").start();
+        // Create a dummy process for stdio mode (use a cross-platform command)
+        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
+        Process process = isWindows
+                ? new ProcessBuilder("cmd", "/c", "type", "NUL").start()
+                : new ProcessBuilder("cat").start();
         try {
             JsonRpcClient client = manager.connectToServer(process, null, null);
             assertNotNull(client);
@@ -126,6 +129,14 @@ void processInfoWithNullPort() {
     // resolveCliCommand is private, so we test indirectly through startCliServer
     // with specific cliPath values.
 
+    // On Windows, "/nonexistent/copilot" is not an absolute path (no drive letter),
+    // so resolveCliCommand wraps it with "cmd /c" and ProcessBuilder.start()
+    // succeeds
+    // (launching cmd.exe). Use a Windows-absolute path to ensure IOException.
+    private static final String NONEXISTENT_CLI = System.getProperty("os.name").toLowerCase().contains("win")
+            ? "C:\\nonexistent\\copilot"
+            : "/nonexistent/copilot";
+
     @Test
     void startCliServerWithJsFile() throws Exception {
         // Using a .js file path causes resolveCliCommand to prepend "node"
@@ -147,8 +158,8 @@ void startCliServerWithJsFile() throws Exception {
     @Test
     void startCliServerWithCliArgs() throws Exception {
         // Test that cliArgs are included in the command
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot")
-                .setCliArgs(new String[]{"--extra-flag"}).setUseStdio(true);
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setCliArgs(new String[]{"--extra-flag"})
+                .setUseStdio(true);
         var manager = new CliServerManager(options);
 
         var ex = assertThrows(IOException.class, () -> manager.startCliServer());
@@ -158,7 +169,7 @@ void startCliServerWithCliArgs() throws Exception {
     @Test
     void startCliServerWithExplicitPort() throws Exception {
         // Test the explicit port branch (useStdio=false, port > 0)
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setUseStdio(false).setPort(9999);
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setUseStdio(false).setPort(9999);
         var manager = new CliServerManager(options);
 
         var ex = assertThrows(IOException.class, () -> manager.startCliServer());
@@ -168,7 +179,7 @@ void startCliServerWithExplicitPort() throws Exception {
     @Test
     void startCliServerWithGitHubToken() throws Exception {
         // Test the github token branch
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setGitHubToken("ghp_test123")
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setGitHubToken("ghp_test123")
                 .setUseStdio(true);
         var manager = new CliServerManager(options);
 
@@ -179,7 +190,7 @@ void startCliServerWithGitHubToken() throws Exception {
     @Test
     void startCliServerWithUseLoggedInUserExplicit() throws Exception {
         // Test the explicit useLoggedInUser=false branch (adds --no-auto-login)
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setUseLoggedInUser(false)
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setUseLoggedInUser(false)
                 .setUseStdio(true);
         var manager = new CliServerManager(options);
 
@@ -190,7 +201,7 @@ void startCliServerWithUseLoggedInUserExplicit() throws Exception {
     @Test
     void startCliServerWithGitHubTokenAndNoExplicitUseLoggedInUser() throws Exception {
         // When gitHubToken is set and useLoggedInUser is null, defaults to false
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setGitHubToken("ghp_test123")
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setGitHubToken("ghp_test123")
                 .setUseStdio(true);
         var manager = new CliServerManager(options);
 
@@ -220,8 +231,7 @@ void startCliServerWithTelemetryAllOptions() throws Exception {
         // so even with a nonexistent CLI path, the telemetry code path is exercised
         var telemetry = new TelemetryConfig().setOtlpEndpoint("http://localhost:4318").setFilePath("/tmp/telemetry.log")
                 .setExporterType("otlp-http").setSourceName("test-app").setCaptureContent(true);
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setTelemetry(telemetry)
-                .setUseStdio(true);
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setTelemetry(telemetry).setUseStdio(true);
         var manager = new CliServerManager(options);
 
         var ex = assertThrows(IOException.class, () -> manager.startCliServer());
@@ -232,8 +242,7 @@ void startCliServerWithTelemetryAllOptions() throws Exception {
     void startCliServerWithTelemetryCaptureContentFalse() throws Exception {
         // Test the false branch of getCaptureContent()
         var telemetry = new TelemetryConfig().setCaptureContent(false);
-        var options = new CopilotClientOptions().setCliPath("/nonexistent/copilot").setTelemetry(telemetry)
-                .setUseStdio(true);
+        var options = new CopilotClientOptions().setCliPath(NONEXISTENT_CLI).setTelemetry(telemetry).setUseStdio(true);
         var manager = new CliServerManager(options);
 
         var ex = assertThrows(IOException.class, () -> manager.startCliServer());

src/test/java/com/github/copilot/sdk/JsonRpcClientTest.java

@@ -135,15 +135,21 @@ void testIsConnectedWithSocketClosed() throws Exception {
 
     @Test
     void testIsConnectedWithProcess() throws Exception {
-        Process proc = new ProcessBuilder("cat").start();
+        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
+        Process proc = isWindows
+                ? new ProcessBuilder("cmd", "/c", "type", "NUL").start()
+                : new ProcessBuilder("cat").start();
         try (var client = JsonRpcClient.fromProcess(proc)) {
             assertTrue(client.isConnected());
         }
     }
 
     @Test
     void testIsConnectedWithProcessDead() throws Exception {
-        Process proc = new ProcessBuilder("cat").start();
+        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
+        Process proc = isWindows
+                ? new ProcessBuilder("cmd", "/c", "type", "NUL").start()
+                : new ProcessBuilder("cat").start();
         var client = JsonRpcClient.fromProcess(proc);
         proc.destroy();
         proc.waitFor(5, TimeUnit.SECONDS);
@@ -155,7 +161,10 @@ void testIsConnectedWithProcessDead() throws Exception {
 
     @Test
     void testGetProcessReturnsProcess() throws Exception {
-        Process proc = new ProcessBuilder("cat").start();
+        boolean isWindows = System.getProperty("os.name").toLowerCase().contains("win");
+        Process proc = isWindows
+                ? new ProcessBuilder("cmd", "/c", "type", "NUL").start()
+                : new ProcessBuilder("cat").start();
         try (var client = JsonRpcClient.fromProcess(proc)) {
             assertSame(proc, client.getProcess());
         }