Add code coverage measurement with Codecov integration

[vgvoleg]

Summary

Introduces code coverage measurement for the ydb package and wires it to Codecov with a PR gate.

• Measurement: pytest-cov (coverage.py) — branch coverage, source=ydb. Auto-generated _grpc/v3..v6 stubs and *_test.py files are excluded; the hand-written _grpc/grpcwrapper/* (topic reader/writer logic) stays measured.
• tox: new cov env — tox -e cov -- ydb / tox -e cov -- tests, producing term/html/xml reports.
• CI: dedicated coverage job (Python 3.12) that runs the unit and integration suites and uploads both to Codecov with unit / integration flags (Codecov merges them).
• Gate (.github/codecov.yml): project coverage may not drop by more than 1%; new code in the diff (patch) must be ≥ 80% covered.
• README: Codecov badge.

Baseline

Local combined run: unit + integration = 81% (605 passed, 18 pre-existing skips). Topic internals are well covered (reader/writer asyncio ~90%, grpcwrapper/ydb_topic.py 87%).

Note on COVERAGE_CORE=sysmon

The coverage job uses the PEP 669 sys.monitoring backend. The default C tracer's overhead breaks the timing-sensitive chaos tests (e.g. test_no_session_leak does acquire(timeout=0.5) then asserts pool state and only restarts YDB after the assert — under tracer slowdown the assert fails, YDB stays down, and the rest of the suite cascades). sysmon keeps overhead low so these pass. It requires Python 3.12+, which is why coverage is collected on a single 3.12 job rather than across the whole matrix.

Follow-up (after this PR runs green)

Add codecov/project and codecov/patch as required status checks in main branch protection to make the gate blocking.

🤖 Generated with Claude Code

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️