Play Framework

test(gradle-plugin): add Gradle TestKit functional tests for the Play1 plugin (PF-139)

The 1830-line Play1Plugin.kt had zero tests; consumer-facing regressions
were only ever verified manually. Wire a JUnit Jupiter 6.0.3 + gradleTestKit()
test source set and add 14 TestKit functional tests covering the high-value,
TestKit-tractable paths: full play1 task-group registration, play1 DSL
wiring, playSecret output contract (64-char [A-Za-z0-9] secret -> certs/.env
under the conf-declared var name), extractPlayModules framework-bundled
directory path (PF-90), and the no-JVM playPid/playStatus pid-file branches.

Test-only: no production source touched. Runs via ./gradlew :gradle-plugin:test
(NOT part of the framework's `ant test`).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>