ci: report Rust coverage to Codecov

[lan17]

Rust CI runs the native, conformance, and real-server tests but does not publish a coverage report. This instruments those existing tests with pinned cargo-llvm-cov tooling and uploads their combined LCOV report to Codecov under the rust flag on pull requests and main.

TypeScript and Go now upload on pull requests as well as main, labeled typescript and go, so Codecov receives all three language reports for the same commit. This preserves the Go coverage collection added on main. All uploads use the existing pinned Codecov action and GitHub OIDC. The Rust LCOV file is also retained as a GitHub Actions artifact for 14 days.

The Rust job retains formatting, clippy, package verification, native tests, committed smoke conformance, the Redis documentation scenario, and Redis/Valkey/Cluster integrations. Package verification runs before instrumentation. Standalone tests/examples, toolchain sources, and the optional test utility runtime are excluded from the report; inline unit-test modules follow LLVM's normal coverage semantics. Doctests still execute, but their coverage is not collected on stable Rust. Full generated conformance remains its separate validation lane.

Validation:

• Ran the instrumented Make/Cargo test sequence locally: formatting, clippy, 155 native tests, 1,738 conformance cases, all five Docker integration tests, and the Redis documentation test passed.
• Exported and inspected LCOV, confirming it contains only the 27 intended Rust source files. The local report records 5,950 of 6,310 executable source lines hit (94.29%); hosted Codecov will establish the CI baseline.
• Verified Rust baseline at 3415e84: 94.29% (5,950/6,310 lines across 27 files). The Rust source and instrumentation are unchanged by the rebase onto Go coverage support.
• Actionlint and git diff --check passed.
• Independent workflow review found no actionable issues.

No Rust implementation or formal specification changes.

— Levicus 🤖

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️