feat: add test coverage report

[xBlaz3kx]

Added test coverage reporting on PR. It will create a comment on the PR with coverage percentage.

I think its important, even though this is a community project, to maintain a high level of test coverage.

[sourcery-ai]

Hey there - I've reviewed your changes - here's some feedback:

• The code_coverage job’s explicit actions/download-artifact step is redundant with fgrosse/go-coverage-report’s built‑in artifact download (via coverage-artifact-name); you can drop the manual download step and let the action handle retrieving coverage.out.
• Changing the default test target to always write coverage.out may be intrusive for local development (e.g., leaving coverage files around or impacting tooling); consider adding a separate test-cover target for CI and keeping test as a plain go test invocation.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `code_coverage` job’s explicit `actions/download-artifact` step is redundant with `fgrosse/go-coverage-report`’s built‑in artifact download (via `coverage-artifact-name`); you can drop the manual download step and let the action handle retrieving `coverage.out`.
- Changing the default `test` target to always write `coverage.out` may be intrusive for local development (e.g., leaving coverage files around or impacting tooling); consider adding a separate `test-cover` target for CI and keeping `test` as a plain `go test` invocation.

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