Add test coverage reporting with grcov and codecov integration

[linw1995]

Summary

This PR introduces comprehensive test coverage reporting to the project using grcov (LLVM-based code coverage) and codecov.io integration. This is the first coverage implementation for the project.

Key Changes

New Coverage Infrastructure

• grcov Integration: Add LLVM-based source coverage analysis with grcov
• Multiple Report Formats: Generate HTML, Cobertura, and Markdown coverage reports
• codecov.io Integration: Automated coverage upload to codecov.io with PR comments and trends
• Development Scripts: New run-cov.sh and run-test.sh scripts for standardized testing workflows

Nix Flake Enhancements

• Add grcov and LLVM tools to development dependencies
• Create dedicated cov and test apps for easy coverage analysis: nix run .#cov and nix run .#test
• Include coverage scripts as packages in development shell
• Maintain full backwards compatibility with existing workflows

Test Infrastructure Improvements

• Simplified Binary Management: Refactor integration test setup to remove complex one-time compilation logic
• Better Target Directory Handling: Support CARGO_TARGET_DIR environment variable for coverage builds
• Improved LSP Reliability: Increase LSP timeouts from 5s to 15s for more stable integration tests
• Cleaner Test Code: Streamline integration test infrastructure and reduce complexity

CI/CD Pipeline Updates

• Replace basic cargo test with coverage-enabled testing in GitHub Actions
• Add codecov upload step with proper token handling and dependabot-safe error handling
• Provide detailed coverage feedback in CI output
• Maintain existing CI reliability while adding coverage metrics

Technical Implementation

Coverage Workflow

# Set LLVM instrumentation flags
export RUSTFLAGS="-Cinstrument-coverage" 
export CARGO_TARGET_DIR="./target/coverage"
export LLVM_PROFILE_FILE="${CARGO_TARGET_DIR}/data/nvim-mcp-%p-%m.profraw"

# Build and run tests with coverage
cargo build --bin nvim-mcp
cargo test

# Generate multi-format reports
grcov ${CARGO_TARGET_DIR}/data \
    --llvm --branch --source-dir . \
    --output-types html,cobertura,markdown

Usage Examples

# Run tests with coverage analysis
nix run .#cov -- --show-output

# Run standard tests  
nix run .#test -- --show-output

# Direct script usage
./scripts/run-cov.sh -- --show-output
./scripts/run-test.sh -- --show-output

Files Added/Modified

New Files

• scripts/run-cov.sh: Comprehensive coverage execution script
• scripts/run-test.sh: Standardized test execution script

Modified Files

• .github/workflows/CI.yaml: Add coverage steps and codecov integration
• flake.nix: Add grcov, LLVM tools, and new coverage/test apps
• src/*/integration_tests.rs: Improve reliability and simplify binary management
• CLAUDE.md, README.md, CHANGELOG.md: Update documentation with coverage instructions

Benefits

• Visibility: Track code coverage across the entire Rust codebase for the first time
• Quality Assurance: Identify untested code paths and improve test comprehensiveness
• Developer Experience: Easy local coverage analysis with HTML reports and terminal summary
• CI Integration: Automated coverage reporting with PR feedback and historical tracking
• Reliability: More stable integration tests with improved timeout handling
• Maintainability: Cleaner test infrastructure with standardized execution scripts

This establishes the foundation for maintaining high code quality and comprehensive test coverage as the project continues to grow.

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage reporting capabilities to the project by integrating codecov.io with cargo-tarpaulin for Rust code coverage analysis. The changes enable automated coverage collection during CI runs and provide developers with a local coverage testing workflow.

Key changes:

• Integration of cargo-tarpaulin for Rust code coverage collection
• Addition of a 'cov' flake app for local coverage testing with proper environment setup
• CI workflow modifications to run tests with coverage and automatically upload results to codecov.io

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
flake.nix	Adds cargo-tarpaulin dependency and creates 'cov' app for coverage testing
.github/workflows/CI.yaml	Updates CI to run coverage tests and upload results to codecov.io
src/server/integration_tests.rs	Modifies integration tests to support tarpaulin compilation with proper flag extraction and target directory handling

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[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 ☂️