ci: diagnose flaky JaCoCo coverage

[warku123]

Purpose

Temporary P0 instrumentation for repeated JaCoCo coverage runs on one fixed commit.

Scope

• adds isolated JaCoCo baseline workflow
• captures exec/XML manifests, semantic coverage counters, JUnit results, Gradle logs, and crash signals
• does not change test parallelism, retries, Gradle configuration, or production/test behavior

Experiment

Re-run the same workflow run 10 times. Compare semantic JaCoCo counters and class-level line coverage; raw exec/XML hashes are evidence only.

Lifecycle

Draft experiment only. Do not merge into develop. Remove diagnostics after evidence collection.

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Summary by cubic

Adds a branch-scoped CI workflow and a Python tool to collect stable JaCoCo coverage evidence and logs so we can compare repeated runs and diagnose flaky coverage. Runs only on the test/flaky-jacoco-coverage branch or via manual dispatch and uploads a manifest and summary with class-level counters and test failures.

• New Features
  • New GitHub Actions workflow “JaCoCo Flake Baseline”: runs clean build, :framework:testWithRocksDb, and jacocoTestReport on JDK 8; gated to the experimental branch.
  • Adds .github/scripts/jacoco_baseline_summary.py to write evidence/summary.json and evidence/manifest.json with report-level counters, class-level line coverage, JUnit totals/failed tests, and Gradle log OOM/crash/retry signals; tolerant of malformed input.
  • Uploads evidence even when Gradle steps fail (then fails the job afterward): JaCoCo .exec and XML, JUnit results, test reports, Gradle logs, JVM crash artifacts, and evidence/metadata.json.
  • No changes to product code, test configs, parallelism, or Gradle settings.

^{Written for commit 33f647e. Summary will update on new commits.}

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/flaky-jacoco-coverage

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