Add code coverage reporting to PR CI

[Copilot]

Adds code coverage collection to PR CI workflow using stable Rust's source-based coverage (available since 1.60.0). Coverage reports are uploaded to Codecov on every PR.

Implementation

• New codecov job in .github/workflows/ci.yaml runs Rust test coverage only
• Uses cargo-llvm-cov with stable toolchain and rust-cache
• Set fail_ci_if_error: false to avoid blocking PRs on Codecov service issues
• 30-minute timeout, respects skip:ci label

Design

The PR job collects only Rust test coverage to minimize build time impact. The existing scheduled cron job continues to provide comprehensive coverage including Python snippets and full test suite.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add code coverage report to CI</issue_title>
<issue_description>https://blog.rust-lang.org/2022/04/07/Rust-1.60.0.html#source-based-code-coverage</issue_description>

Comments on the Issue (you are @copilot in this section)

@coolreader18 Yeah I saw that; that's exciting! @youknowone I was forgetting we had one in cron. Can we make report using codecov? https://about.codecov.io/language/rust/ @youknowone #3649 made the first report: https://app.codecov.io/gh/RustPython/RustPython @youknowone I looked at it and I like it. If it doesn't increase build time that much (because now we can do it with stable compiler), adding it to PR CI will be the final step.

• Fixes Add code coverage report to CI #3588

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