Add ValidatesRunner cross-SDK test coverage comparison

[damccorm]

Summary

• Adds .test-infra/compatCoverage/compat-test-comparison.md comparing the validates-runner test suite coverage between the Java and Python SDKs
• Table covers 100+ test cases across 14 feature categories: ParDo, State & Timers, GroupByKey, Flatten, Combine, CoGroupByKey, Side Inputs, Windowing, Metrics, TestStream, Impulse, Splittable DoFn, Reshuffle, Redistribute, DoFn Lifecycle, Timestamps, Per-key Ordering, and Cross-Language (XLang) transforms
• Each row links directly to the relevant test in Java (@Category(ValidatesRunner.class)) and the closest Python equivalent (in ptransform_test.py, sideinputs_test.py, combiners_test.py, fn_runner_test.py, metric_test.py, dofn_lifecycle_test.py, and validate_runner_xlang_test.py)
• Includes an Observations section summarizing key coverage gaps (e.g., Java-only: TestStream, Redistribute, per-key ordering, set/map state; near-parity: XLang transforms)

Test plan

• Verify all Java SDK links resolve to the correct test method and line number in the repo
• Verify all Python SDK links resolve to the correct test method and line number in the repo
• Confirm the Observations section accurately reflects the gap analysis

🤖 Generated with Claude Code