[Rishavbhattarai]

Fixes #18691.

Ports SplittableParDoProcessFnTest off the deprecated DoFnTester.

These tests drive ProcessFn by hand: they advance processing time, fire its timers and check the watermark hold between calls. TestPipeline can't do that, so I kept the ProcessFnTester helper and changed what's inside it:

• ProcessFn now runs through DoFnRunners.simpleRunner, the same runner SplittableProcessElementsEvaluatorFactory uses in the DirectRunner. StepContext hands it the test's existing InMemoryTimerInternals and TestInMemoryStateInternals.
• close() calls finishBundle() and then ProcessFn.tearDown(), the same order DoFnTester.close() used.
• Outputs from the splittable invoker go into a plain list. takeOutputElements() and peekOutputElementsInWindow() read from that list, so none of the test bodies change.
• Removed DoFnTesterWindowedValueReceiver and the DoFnTester import.

Testing: ./gradlew :runners:core-java:test --tests SplittableParDoProcessFnTest passes (15/15), and spotlessCheck is clean.

[github-actions]

📋 Review Summary

This PR cleanly migrates SplittableParDoProcessFnTest off the deprecated DoFnTester by updating the ProcessFnTester test helper to execute ProcessFn directly via DoFnRunners.simpleRunner. The implementation preserves bundle lifecycle management, state/timer handling, and output verification without needing to alter any of the existing test cases.

🔍 General Feedback

• The setup of DoFnRunners.simpleRunner and custom WindowedValueMultiReceiver accurately mirrors how ProcessFn is invoked in runners like DirectRunner.
• Bundle lifecycle transitions (setup, startBundle, finishBundle, tearDown) are properly preserved in ProcessFnTester.

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[github-actions]

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