ci: add Codecov coverage for Go SDK (#2773)

[atharvalade]

Which issue does this PR close?

Closes #2773

Rationale

Rust and Java already upload coverage to Codecov — Go SDK was generating coverage but never uploading it.

What changed?

Coverage was already produced (reports/go-coverage.out) by the pre-merge action but never sent to Codecov. Added a codecov/codecov-action@v5 upload step in _test.yml for sdk-go, a go-coverage baseline job in post-merge.yml following the Java pattern (start server, run tests with coverage, upload), and registered the go flag in codecov.yml while removing foreign/go/** 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 three files
4. Yes

[atharvalade]

can someone look into this Rust integration test (connectors::postgres::postgres_source::processed_column_source_marks_rows_after_producing), not related to the Go codecov changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.74%. Comparing base (cca86fe) to head (db232ed).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2793      +/-   ##
============================================
- Coverage     68.47%   64.74%   -3.73%     
  Complexity      656      656              
============================================
  Files           852      900      +48     
  Lines         68221    72576    +4355     
  Branches      59682    59692      +10     
============================================
+ Hits          46714    46989     +275     
- Misses        19177    23240    +4063     
- Partials       2330     2347      +17     

Flag	Coverage Δ
csharp	67.06% <ø> (-0.19%)	⬇️
go	6.84% <ø> (?)
java	52.19% <ø> (ø)
rust	69.62% <ø> (-0.03%)	⬇️

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see 62 files with indirect coverage changes

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[hubcio]

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

[atharvalade]

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

added