Integrate code coverage with Makefile targets, nightly CI, and Codecov

[Copilot]

This PR integrates comprehensive code coverage into the OpenMina repository, following the approach used in proof-systems#2504.

Features Added

🎯 Makefile Targets

• make setup-coverage-tools - Install required coverage tools (llvm-tools-preview, grcov)
• make test-coverage - Run basic tests with coverage (libraries and tests only)
• make test-with-coverage - Run comprehensive tests with coverage (includes binaries)
• make coverage-report - Generate HTML coverage report
• make coverage-lcov - Generate LCOV report for CI/codecov
• make coverage-summary - Display coverage summary in terminal
• make coverage-clean - Clean all coverage data and reports

🌙 CI Integration

• Nightly workflow that runs coverage analysis at 2:00 AM UTC daily
• Manual trigger with choice between basic/comprehensive coverage
• Automatic upload to Codecov with proper token handling
• Artifact preservation for coverage reports (30-day retention)
• Coverage summary in GitHub step summary

⚙️ Configuration

• codecov.yml with appropriate ignore patterns and thresholds
• Coverage targets: 70% minimum, 85% target, 95% excellent
• Integration with GitHub checks and annotations
• Proper exclusions for test files, generated code, and tooling

📚 Documentation

• Comprehensive coverage guide at website/docs/developers/code-coverage.md
• Usage examples and troubleshooting guide
• IDE integration instructions (VS Code Coverage Gutters)
• Best practices for writing testable code
• Added to developer documentation sidebar

Usage Examples

# Quick setup
make setup-coverage-tools

# Fast coverage during development
make test-coverage
make coverage-summary

# Comprehensive coverage for analysis
make test-with-coverage
make coverage-report
open target/coverage/html/index.html

# Generate reports for CI
make coverage-lcov

# Clean up
make coverage-clean

Verification

The implementation has been tested end-to-end:

• ✅ Coverage data collection works correctly
• ✅ HTML and LCOV report generation verified
• ✅ Coverage summary displays properly
• ✅ All Makefile targets functional
• ✅ Documentation accessible and complete

Example coverage output from testing:

| file                            | coverage | covered   | missed_lines      |
|---------------------------------|----------|-----------|-------------------|
| macros/src/action_event.rs      | 0.00%    | 0 / 396   | 19-495            |
| macros/src/lib.rs               | 33.33%   | 3 / 9     | 3-9               |
| macros/src/serde_yojson_enum.rs | 98.37%   | 241 / 245 | 13, 118, 175, 209 |

Total coverage: 37.54%

This provides a solid foundation for tracking and improving code coverage across the OpenMina codebase.

Fixes #1197.

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:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (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 the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.