ci: add Codecov coverage for Python SDK

[atharvalade]

Which issue does this PR close?

Closes #2775

Rationale

Rust and Java already upload coverage to Codecov, Python SDK was missing.

What changed?

Added pytest-cov to Python dev deps and --cov flags to pytest calls in the pre-merge action. Added Codecov upload steps in _test.yml (pre-merge) and a python-coverage baseline job in post-merge.yml, following the same pattern as Java. Registered the python flag in codecov.yml and removed foreign/python/** from the ignore list.

Local Execution

• CI-only workflow changes
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. used to scaffold the workflow YAML following existing Java/Rust patterns
3. Verified by diffing against existing Java coverage steps and reviewing all five files
4. Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.42%. Comparing base (9541332) to head (88215cd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2790      +/-   ##
============================================
- Coverage     68.44%   68.42%   -0.03%     
  Complexity      656      656              
============================================
  Files           852      852              
  Lines         68196    68196              
  Branches      59657    59667      +10     
============================================
- Hits          46678    46660      -18     
- Misses        19187    19195       +8     
- Partials       2331     2341      +10     

Flag	Coverage Δ
csharp	67.06% <ø> (-0.19%)	⬇️
java	52.19% <ø> (ø)
rust	69.60% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 11 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[hubcio]

The CODECOV_TOKEN secret is not passed to test-python in pre-merge.yml, so the upload will silently fail. Need to add secrets: CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }} to the test-python job.

[atharvalade]

The CODECOV_TOKEN secret is not passed to test-python in pre-merge.yml, so the upload will silently fail. Need to add secrets: CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }} to the test-python job.

added