[python] Batch mixed Java write tests

[JingsongLi]

Summary

Batch the Java write side of run_mixed_tests.sh by module before the existing Python validation flow. This reduces repeated Maven invocations while preserving an opt-out path for the original one-by-one execution.

Changes

• Add a batched Maven helper that groups Java write tests per module with the existing Python-version gates.
• Skip duplicate Java write Maven calls in later Java-to-Python phases after the batched phase succeeds.
• Stop early on batched Java write failure and document MIXED_TESTS_BATCH_JAVA_WRITES=false for isolating failures with the previous flow.

Testing

• bash -n paimon-python/dev/run_mixed_tests.sh
• git diff --check
• Full mixed Java/Python E2E script or CI run

Notes

I attempted a small Maven multi-method selection smoke test, but it produced no output for over two minutes and was interrupted to avoid leaving a long-running local verification.

[leaves12138]

LGTM. I reviewed the batching control flow for the mixed Java/Python script.

The default path now runs the Java write methods in per-module batches before the existing Python validation flow, and the later Java-write phases are skipped only after the batched phase succeeds. The MIXED_TESTS_BATCH_JAVA_WRITES=false opt-out still preserves the original one-by-one Maven flow.

Local verification:

• bash -n paimon-python/dev/run_mixed_tests.sh
• git diff origin/master..HEAD --check
• Mocked full-script dry run with fake mvn/python: default batch path reduced Maven calls from 31 to 14 while keeping the Java read phases, and the opt-out path kept the original 31 Maven invocations.
• Mocked failed batched Java write path: the script exits before entering Python validation.

I did not run the full mixed Java/Python E2E suite locally.