fix(test-plans): use self-contained Maven fixture for resolve-unknown-type plan

[wenytang-ms]

What & Why

The java-maven-resolve-type E2E plan has been failing in scheduled runs (e.g. actions/runs/25419116469). Investigation showed:

• Old fixture: redhat-developer/vscode-java's salut Maven project, which sets <source>1.7</source> / <target>1.7</target>.
• CI runners only install JDK 21.
• Under that mismatch JDT reports the pom compliance warnings ("compiler compliance 1.7 but JRE 11 is used") and shows Java: Ready, but never publishes semantic diagnostics for Foo.java. So Gson gson; does not produce a Gson cannot be resolved to a type error and navigateToError / applyCodeAction "Resolve unknown type" cannot proceed.
• Older @vscjava/vscode-autotest masked this with a silent-fallback in applyCodeAction (Enter on whatever the first action happened to be). Once that fallback was removed upstream, the plan started consistently failing on [check-code-action] applyCodeAction "Resolve unknown type" with Available actions: Extract interface..., Source Actions....

Verified locally: bumping the pom to <release>11</release> makes JDT publish the unresolved-type diagnostic in ~8 s and the full plan goes from 4/6 → 6/6 passed.

Change

Add a small, self-contained Maven fixture under test-fixtures/maven-resolve-type/ that targets JDK 11 explicitly, and re-point the test plan at it:

• test-fixtures/maven-resolve-type/pom.xml — maven-compiler-plugin 3.8.0, <release>11</release>, no extra deps.
• test-fixtures/maven-resolve-type/src/main/java/com/example/App.java — minimal class.
• test-fixtures/maven-resolve-type/README.md — explains why the fixture lives here and the JDK-compliance pitfall.
• test-plans/java-maven-resolve-type.yaml — workspace repointed; opens App.java; inserts Gson gson; at line 4 (inside the class body); rest of the steps unchanged.

Why a local fixture rather than reusing salut-java11

Owning the fixture in this repo decouples this plan from upstream vscode-java test-resource changes and removes the implicit ordering dependency of git clone vscode-java finishing before this plan can run.

Verification

Local run with @vscjava/vscode-autotest (with the upcoming navigateToError fix that fails fast instead of falling through to warnings):

✅ [ls-ready] waitForLanguageServer (55088ms)
✅ [open-app] open file App.java (1299ms)
✅ [insert-unknown-type] insertLineInFile ... (3974ms)
✅ [navigate-to-error] navigateToError 1 (8083ms)
✅ [check-code-action] applyCodeAction Resolve unknown type (2559ms)
✅ [save-after-resolve] saveFile (970ms)
📊 Results: 6/6 passed

Problems panel screenshot confirms Gson cannot be resolved to a type Java(16777218) is published on App.java:4, which is the diagnostic code the vscode-maven Resolve unknown type Code Action keys on.