Play Framework

fix(invoker): retire invokeWithContinuation + CONTINUATIONS_STORE plumbing

Step 2/4 toward eliminating Javaflow. After commit 1 collapsed Controller.await(...)
to inline-block, the Javaflow continuation dispatch in ActionInvoker is dead code:
no caller ever pushes a Continuation/StackRecorder onto Request.args, no callback
gets stashed under ActionInvoker.A, no future under ActionInvoker.F.

Removes from ActionInvoker:
- Imports for org.apache.commons.javaflow.Continuation and StackRecorder
- The invokeWithContinuation(...) method (~70 lines) — both the
  callback-resume branch and the StackRecorder.registerThread/Suspend dispatch
- The constants C, A, F, CONTINUATIONS_STORE_LOCAL_VARIABLE_NAMES,
  CONTINUATIONS_STORE_RENDER_ARGS, CONTINUATIONS_STORE_PARAMS,
  CONTINUATIONS_STORE_VALIDATIONS, CONTINUATIONS_STORE_VALIDATIONPLUGIN_KEYS
- The invoke(...) action-method special case — now invokes the method
  reflectively like every other call

isActionMethod is kept; it's still used by findActionMethod to filter
@Before/@After/@Finally/@Catch/@Util interceptors during route resolution.

ValidationPlugin.beforeActionInvocation no longer checks for
CONTINUATIONS_STORE_VALIDATIONS — the check existed solely to short-circuit
on the second pass through beforeActionInvocation that Javaflow caused on
await() resume, which can no longer happen with await as inline-block.
A comment marks the historical behavior in case anyone wonders why this
plugin now does less work than it did three commits ago.

ant unittest passes.

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