Play Framework

chore(docs,tests): retire VT toggle docs + clean up test setProperty noise

Step 5/5 — finishes the VT-only modernization.

Docs:
- CLAUDE.md: replace the "Virtual Threads (opt-in)" section with a single
  paragraph saying VT is unconditional, the legacy toggles are gone, and
  Play.java warns at boot if any of the keys are still in application.conf.
- AGENTS.md: same update.

Tests:
- Drop the now-no-op Play.configuration.setProperty("play.threads.virtual",
  "true") (and per-subsystem variants) calls from InvokerVirtualThreadTest,
  JobsPluginVirtualThreadTest, MailVirtualThreadTest, VirtualThreadPinningTest.
  The toggle is no longer wired to anything — these calls were just clutter
  after commit 4. The tests still drive Invoker.init() / plugin.onApplicationStart()
  / etc. to set up the executor, which is the actual entry point.
- The "false-toggle ignored" tests in InvokerVirtualThreadTest and
  JobsPluginVirtualThreadTest deliberately keep the setProperty calls (they
  document that the legacy keys are no-ops).
- VirtualThreadIntegrationTest's class-level javadoc rewritten to drop the
  "when play.threads.virtual=true is set" framing.

ant test passes.

Net pass numbers (Javaflow elimination + VT-only):
- ~1100 lines deleted
- 8 source files removed (Javaflow enhancers, exception types,
  ExecutorFacade, VirtualThreadConfig, related tests)
- 1 native jar removed (commons-javaflow)
- 2 enhancers retired from the classloader pipeline

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>