Tidy the bridgeless runtime executor plumbing (#58325)

[javache]

Summary:

Follow-ups to the buffered CallInvoker, kept out of that diff so the behavioural
change stays reviewable on its own. No behaviour change intended here.

• BufferedRuntimeExecutor gains asRuntimeExecutor() and
  asWeakRuntimeExecutor(). RuntimeExecutor is a std::function, so callers
  cannot hand it the object and instead each hand-rolled the same capturing
  lambda; the ownership question now has one answer per lifetime policy. The
  weak form is what getBufferedRuntimeExecutor needs, which must not keep the
  instance alive.
• BufferedWork becomes private. It was only ever an implementation detail, and
  being public put its fields in the C++ API snapshot.
• The buffer holds a std::vector sorted once at flush rather than a
  std::priority_queue. Indices are handed out before the lock, so arrival
  order can differ from submission order and something has to restore it — but a
  heap is the expensive way. std::priority_queue::top() returns a const
  reference, so every flushed item copied its std::function on the way out;
  that copy is now a move, and nothing sifts on the way in.
• runtimeExecutorThatGoesThroughRuntimeScheduler was a second copy of
  getUnbufferedRuntimeExecutor(); it now calls it.
• RuntimeSchedulerCallInvoker is marked deprecated. Its remaining users are
  migrated: ReactCxxPlatform holds a ReactInstance and can use
  createJSCallInvoker(), and four other files only #included it without ever
  constructing one. The single remaining use is the flag-off branch, which is
  suppressed locally and goes when the flag is cleaned up.
• Drops a null check on bufferedRuntimeExecutor_ in callFunctionOnModule
  that the constructor makes unreachable.

Changelog:
[Internal]

Reviewed By: christophpurrer

Differential Revision: D118616045

[meta-codesync]

@javache has exported this pull request. If you are a Meta employee, you can view the originating Diff in D118616045.