ci(mcp-diff): build UI artifacts available to baseline checkout (#2523)

The mcp-server-diff action checks the baseline ref out into a separate
working directory and runs install_command there. Without prebuilt UI
artifacts, pkg/github/ui_dist/ is empty on the baseline side and
UIAssetsAvailable() returns false, producing a false-positive diff that
"adds" _meta.ui to MCP Apps tools on every PR.

Stash the artifacts to RUNNER_TEMP after the workflow's build-ui step,
then restore them from install_command so both the baseline and PR
checkouts register identical MCP Apps UI metadata.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums

.github/workflows/mcp-diff.yml

@@ -27,6 +27,16 @@ jobs:
       - name: Build UI
         uses: ./.github/actions/build-ui
 
+      - name: Stash UI artifacts for baseline checkout
+        # mcp-server-diff checks the baseline ref out into a separate working
+        # directory and runs install_command there. Without these prebuilt
+        # artifacts, pkg/github/ui_dist/ would be empty on the baseline side
+        # and UIAssetsAvailable() would return false, producing a false-positive
+        # diff that "adds" _meta.ui to MCP Apps tools on every PR.
+        run: |
+          mkdir -p "${RUNNER_TEMP}/ui_dist"
+          cp pkg/github/ui_dist/*.html "${RUNNER_TEMP}/ui_dist/"
+
       - name: Generate diff configurations
         id: configs
         # The generator imports pkg/github so any new entry in
@@ -43,7 +53,10 @@ jobs:
         uses: SamMorrowDrums/mcp-server-diff@v2.3.5
         with:
           setup_go: "false"
-          install_command: go mod download
+          install_command: |
+            go mod download
+            mkdir -p pkg/github/ui_dist
+            cp "${RUNNER_TEMP}"/ui_dist/*.html pkg/github/ui_dist/
           start_command: go run ./cmd/github-mcp-server stdio
           env_vars: |
             GITHUB_PERSONAL_ACCESS_TOKEN=test-token
@@ -79,6 +92,13 @@ jobs:
       - name: Build UI
         uses: ./.github/actions/build-ui
 
+      - name: Stash UI artifacts for baseline checkout
+        # See the stdio job above for rationale: the action's baseline checkout
+        # has no UI artifacts unless we hand them over via RUNNER_TEMP.
+        run: |
+          mkdir -p "${RUNNER_TEMP}/ui_dist"
+          cp pkg/github/ui_dist/*.html "${RUNNER_TEMP}/ui_dist/"
+
       - name: Generate diff configurations
         id: configs
         # See script/print-mcp-diff-configs/main.go. The http-headers variant
@@ -97,7 +117,10 @@ jobs:
         uses: SamMorrowDrums/mcp-server-diff@v2.3.5
         with:
           setup_go: "false"
-          install_command: go mod download
+          install_command: |
+            go mod download
+            mkdir -p pkg/github/ui_dist
+            cp "${RUNNER_TEMP}"/ui_dist/*.html pkg/github/ui_dist/
           http_start_command: go run ./cmd/github-mcp-server http --port 8082
           http_startup_wait_ms: "5000"
           configurations: ${{ steps.configs.outputs.configurations }}