Play Framework

fix(invoker,jobs,mail): make virtual-thread execution unconditional

Step 1/5 toward VT-only. Drops the if (VirtualThreadConfig.is*Enabled())
branches in Invoker.init / Invoker.ensureExecutor / JobsPlugin.onApplicationStart
/ Mail.getExecutor. Each subsystem now constructs a VirtualThreadScheduledExecutor
(or VT-named ExecutorService for Mail) unconditionally.

Operator note: the play.threads.virtual* configuration keys still exist as
this commit just collapses the runtime branches; the keys become no-ops.
A startup WARN that flags any leftover legacy values follows in commit 4.

The deprecated mirror fields (Invoker.executor, Invoker.virtualExecutor,
Invoker.usingVirtualThreads, JobsPlugin equivalents) stay populated for one
transition step so binary-compatible plugins still see something non-null.
They go in commit 4.

Test fixups in this commit (kept atomic per commit so bisects stay green):
- InvokerVirtualThreadTest: collapse the four init-mode tests into one that
  asserts VT is selected unconditionally. Drops platform-mode assertions.
- VirtualThreadIntegrationTest: drop the platformThreadModeStillWorks test.
- JobsPluginVirtualThreadTest: collapse two init-mode tests into one
  unconditional-VT assertion.

ant test passes.

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