Play Framework

fix(boot,test): warn on retired threading keys + VT-back functional test serializer

Two related VT-purity follow-ups:

1. Boot warn-block now flags raw retired threading keys regardless of the
   active profile. Previously it scanned Play.configuration.keySet(), which
   the two-pass profile filter in readOneConfigurationFile had already
   stripped: a %prod.play.pool=10 sitting in application.conf during a dev
   run was silently dropped, so the operator never learned the knob was
   retired until they next switched profiles. New side-channel set
   (retiredThreadingKeys) is populated during raw config load, preserving
   the original key form (with any %<id>. prefix) so the WARN points
   straight at the offending line. Also extends the watched-keys list to
   play.pool and play.jobs.pool (the prior commit only caught
   play.threads.virtual*).

2. TestEngine.functionalTestsExecutor switched from
   Executors.newSingleThreadExecutor() to newSingleThreadExecutor with a
   VirtualThreadFactory("functional-test"). Eliminates the last
   platform-thread executor in framework code; serialization semantics are
   preserved (the queue, not the carrier kind, is what serializes).
   Functional tests still need single-threaded dispatch because
   FunctionalTest.savedCookies and renderArgs are static.

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