Add the code coverage job

[tian-lt]

What

Adds a dedicated code coverage job to the CI pipeline. It builds the tests with GCC coverage instrumentation, runs both the hosted and freestanding test suites, generates a coverage report scoped to the public headers (include/proxy/), publishes a summary to the workflow run page, and uploads the full report as an artifact.

Samples

lines: 99.0% (483 out of 488)
functions: 24.6% (7784 out of 31594)
branches: 76.3% (100 out of 131)

[Copilot]

Pull request overview

Adds a dedicated code coverage build/test path and wires it into the existing CI pipeline so coverage reports can be generated and uploaded as artifacts.

Changes:

• Added CMake configure/build/test presets for coverage (hosted + freestanding).
• Introduced a reusable GitHub Actions workflow to run coverage-instrumented tests and generate a gcovr report.
• Hooked the new coverage workflow into the main CI pipeline.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
CMakePresets.json	Adds coverage and coverage-freestanding configure/build/test presets to enable coverage-instrumented builds.
.github/workflows/pipeline-ci.yml	Invokes the new code coverage BVT workflow as an additional CI job.
.github/workflows/bvt-codecoverage.yml	New reusable workflow that builds/tests with coverage and uploads gcovr reports as an artifact.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[mingxwa]

Shall we count coverage of freestanding separately?

[mingxwa]

Shall we use GitHub integration actions/upload-code-coverage?

[mingxwa]

I reviewed the generated coverage.function.html, looks like most "uncovered" functions are just template instantiations from unit tests/doc examples/benchmarks. The more instantiations there are, the less function coverage there will be. I think we should turn off function coverage if possible - it's meaningless for a template library.