test(oss-licenses): add standalone testapp for plugin verification

[timothyfroehlich]

Summary

Introduces a standalone Android test application for verifying the oss-licenses plugin end-to-end, along with CI jobs to run it on every PR.

• Standalone testapp (oss-licenses-plugin/testapp/) — a real Android app that applies the plugin, declares dependencies with known licenses (Gson, Guava), and runs Robolectric tests against both V1 and V2 license display activities.
• includeBuild for local dev — by default, cd testapp && ./gradlew build builds the plugin from source via Gradle composite builds. No manual ./gradlew publish needed. CI and e2e tests pass -PusePublishedPluginFrom=../build/repo to use a pre-published artifact instead.
• Configuration cache strictness — the testapp enables STABLE_CONFIGURATION_CACHE, org.gradle.configuration-cache=true, and org.gradle.configuration-cache.problems=fail in gradle.properties.
• CI jobs — three new jobs in the workflow:
  • oss-licenses-build — builds and publishes the plugin (pulled out of the matrix)
  • oss-licenses-testapp — builds the testapp with the published plugin
  • oss-licenses-testapp-cc — configuration cache verification (build → clean → rebuild). This job uses continue-on-error: true because the plugin currently has a known CC bug (refactor(oss-licenses): replace eager dependency resolution with lazy ArtifactView providers #381) that will be fixed in a follow-up PR.
• Housekeeping — adds gradle-daemon-jvm.properties (JDK 21), foojay toolchain resolver, .DS_Store to .gitignore.

Test plan

• cd oss-licenses-plugin/testapp && ./gradlew build succeeds locally (uses includeBuild)
• cd oss-licenses-plugin && ./gradlew publish && cd testapp && ./gradlew build -PusePublishedPluginFrom=../build/repo succeeds (published path)
• CI oss-licenses-testapp job passes
• CI oss-licenses-testapp-cc job fails as expected (known CC bug, continue-on-error prevents blocking)
• CI oss-licenses-build and build (other plugins) jobs pass

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