Replace Codecov with GitHub native code coverage

[glen-84]

Summary

• Replaces the Codecov upload in both CI workflows (ci.yml for PRs, coverage.yml for main) with GitHub's native code coverage: the per-project Cobertura reports are merged with dotnet-coverage merge and the single result is uploaded via actions/upload-code-coverage (SHA-pinned), with code-quality: write scoped to the upload-coverage job.
• Deletes .github/codecov.yml and drops the CODECOV_TOKEN usage. The test matrix and coverage collection are unchanged.
• Fork-PR coverage is intentionally not carried over: GitHub's native feature does not support PRs from forks yet (acknowledged upstream as a roadmap item).

Test plan

• Validated the merge locally against a real coverage.yml run's 125 coverage-* artifacts: dotnet-coverage 18.8.0 merge … -f cobertura produced one valid Cobertura report (150 packages, 13,642 classes, line-rate ~0.49).
• Workflows pass YAML parsing and actionlint (the code-quality permission-scope warning is a known actionlint gap for this preview feature).
• After merge, requires "Code Quality" enabled on the repo; then the Upload Coverage job should be green and github-code-quality[bot] should post a coverage summary on PRs. The CODECOV_TOKEN secret can then be deleted.

[Copilot]

Pull request overview

This PR migrates CI coverage reporting from Codecov to GitHub’s native code coverage feature by merging per-project Cobertura reports into a single report and uploading it via actions/upload-code-coverage.

Changes:

• Replaces Codecov upload steps in both PR CI (ci.yml) and main-branch coverage (coverage.yml) with a dotnet-coverage merge + actions/upload-code-coverage flow.
• Adds scoped job permissions for the upload job (code-quality: write) and removes reliance on CODECOV_TOKEN.
• Removes the Codecov configuration file (.github/codecov.yml).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
.github/workflows/coverage.yml	Merge Cobertura artifacts on main pushes and upload merged report to GitHub code coverage with scoped permissions.
.github/workflows/ci.yml	Same coverage merge + GitHub upload flow for PR CI runs, replacing Codecov usage.
.github/codecov.yml	Deleted since Codecov is no longer used.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the gai/github-native-co... branch is 49%. Coverage data for the main branch is not yet available.

Show a code coverage summary of the most covered files.

File	main	gai/github-native-co... c2c213e	+/-
/home/runner/wo.../FieldResult.cs	—	100%	—
/home/runner/wo...SchemaMerger.cs	—	98%	—
/home/runner/wo...xecutionTree.cs	—	92%	—
/home/runner/wo...ationPlanner.cs	—	88%	—
/home/runner/wo...mentRewriter.cs	—	87%	—
/home/runner/wo...eBuilderBase.cs	—	85%	—
/home/runner/wo...xGenerator.g.cs	—	80%	—
/home/runner/wo...hResultStore.cs	—	80%	—
/home/runner/wo...xGenerator.g.cs	—	71%	—
/home/runner/wo...lient.Client.cs	—	1%	—

---

_{Code Coverage is in Public Preview. Learn more and provide us with your feedback.}