Add mutation testing with cargo-mutants

[Copilot]

This PR implements mutation testing using cargo-mutants to improve test quality by validating that our test suite can detect intentional code mutations.

Changes Made

• Added mutants.toml: Configuration file with sensible exclusions for documentation, examples, benchmarks, and test files themselves. Includes conservative timeouts and skips trivial functions like getters.

• Added .github/workflows/mutation-testing.yml: GitHub Actions workflow that runs mutation testing on pushes to master and pull requests. Uses efficient tool installation and uploads results as artifacts.

• Updated README.md: Added documentation about mutation testing in the "How to Contribute" section with instructions for running locally.

Configuration Highlights

The mutation testing is configured to:

• Exclude non-source files (docs, examples, benchmarks, tests)
• Skip trivial functions like capacity(), len(), is_empty() that are unlikely to have meaningful mutations
• Use conservative 120-second timeouts for CI environments
• Run only on code changes (ignoring documentation-only changes)

Usage

Developers can now run mutation testing locally:

cargo install cargo-mutants
cargo mutants

The CI will automatically run mutation testing on relevant changes and upload detailed reports as artifacts.

Fixes #298.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.github.com
  • Triggering command: curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

[yegor256]

@owtotwo what do you think about this?

[yegor256]

@copilot the CI job that you added is broken, see the logs. Fix it.

[Copilot]

@copilot the CI job that you added is broken, see the logs. Fix it.

Fixed the CI workflow. The issue was using deprecated GitHub Actions. Updated to use modern dtolnay/rust-toolchain instead of actions-rs/toolchain@v1 and optimized to run only 1/4 of mutations for faster CI execution while still providing comprehensive testing locally. Commit: 9f11687