fix: Add deprecation warning for removed flushSync API

[gabrielfranca95]

Summary

I am submitting this PR to address issue #35424 regarding the silent removal of flushSync. As pointed out in the issue, the renaming of flushSync to flushSyncFromReconciler in PR #28500 was a breaking change that caused flushSync to become undefined without warning, breaking custom renderers like @opentui/react.

To fix this usability issue, I have:

1. Re-introduced flushSync as a deprecated stub (placeholder function) in ReactFiberReconciler.js.
2. Added a runtime warning that I configured to trigger only when the function is called (and only once per session in __DEV__).
3. Provided clear guidance in the warning message to use updateContainerSync() followed by flushSyncWork(), as requested in the issue.

This ensures that I help existing custom renderer authors receive a helpful error message instead of failing silently with undefined errors.

Closes #35424

How did you test this change?

I added a new test file: packages/react-reconciler/src/tests/ReactReconcilerFlushSyncDeprecation-test.js to specifically verify the deprecation logic I implemented.

Verification Steps:

1. Automated Testing: I ran the new test via yarn test ReactReconcilerFlushSyncDeprecation.
   • I verified that Reconciler.flushSync is exported as a function.
   • I verified that calling flushSync() emits the expected console.error warning.
   • I verified that the warning includes the migration instruction: "Use updateContainerSync() followed by flushSyncWork() instead."
2. Correctness: I ensured the warning does not fire on module load (import), but only upon execution, preventing noise for users who might import the reconciler but not use this specific method.

Test Output:
PASS packages/react-reconciler/src/tests/ReactReconcilerFlushSyncDeprecation-test.js
ReactReconciler flushSync deprecation
✓ warns when accessing the deprecated flushSync export (172 ms)

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[ZoroCWS]

/ask rate this PR quality on a scale of 1 to 10 and give me the examples

[ZoroCWS]

/ask rate this PR quality on a scale of 1 to 10 and give me the examples