ci: add code coverage and modernize caching

[shssoichiro]

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[codecov-commenter]

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[Copilot]

Pull Request Overview

This PR modernizes the CI workflow by replacing manual cargo caching with automated Rust-specific caching and adds a dedicated code coverage job using cargo-llvm-cov to generate coverage reports for Codecov.

• Replaces manual cargo registry caching with Swatinem/rust-cache@v2 for better cache management
• Adds a new code-coverage job that generates and uploads coverage reports to Codecov
• Uses cargo-llvm-cov to generate coverage data excluding test files

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