[harshilv17]

Fixes #18688.

Ports the Elasticsearch write tests in ElasticsearchIOTestCommon off the deprecated DoFnTester.

TestPipeline doesn't fit here either: as the existing comments say, bundle size is the runner's decision, and testWriteWithMaxBatchSize / testWriteWithMaxBatchSizeBytes check how many documents Elasticsearch holds between elements of the same bundle.

So, like the DatastoreV1Test (#40866) and BatchViewOverridesTest (#41202) ports, the four tests now use a small BulkIOBundleFnRunner that runs BulkIOBundleFn in-process through DoFnRunners.simpleRunner. What differs from those helpers:

• BulkIOBundleFn creates its REST client in @Setup and closes it in @Teardown, so the helper invokes both through DoFnInvokers.
• Like DoFnTester, a bundle starts on the first element and finishes at the end of processBundle or on close(), so the tests that feed elements one at a time keep their semantics.
• It unwraps UserCodeException, so testWriteWithErrors still matches the fn's own exception message.

elasticsearch-tests-common gains an implementation dependency on :runners:core-java for DoFnRunners. The test assertions are unchanged.

Ran locally against Elasticsearch 8 (testcontainers): testWriteWithErrors, testWriteWithAllowableErrors, testWriteWithMaxBatchSize and testWriteWithMaxBatchSizeBytes pass. spotlessApply and checkstyleMain are clean.

• Small/focused contribution (ICLA not required).
• Mentions the issue (Fixes #18688).
• CHANGES.md: not needed, test-only change.

[github-actions]

📋 Review Summary

This PR cleanly ports the four BulkIO.BulkIOBundleFn tests in ElasticsearchIOTestCommon off the deprecated DoFnTester by introducing a focused BulkIOBundleFnRunner helper backed by DoFnRunners.simpleRunner and DoFnInvokers. Lifecycle handling (@Setup, @StartBundle, @FinishBundle, and @Teardown in a finally block) and exception unwrapping preserve existing test semantics.

🔍 General Feedback

• Calling invoker.invokeTeardown() inside a finally block in BulkIOBundleFnRunner.close() is a nice improvement over DoFnTester, ensuring the Elasticsearch RestClient is always closed even when finishBundle() throws an exception (such as in testWriteWithErrors).
• The changes are well-scoped and consistent with similar DoFnTester migration PRs.

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[github-actions]

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