Improve builder-rs: safety, documentation, and test coverage

[Copilot]

🚀 Pull Request

📋 Description

Code quality improvements for the builder-rs Rust service. Eliminates panic-prone patterns, adds documentation and test coverage for critical utilities.

Safety & Error Handling

• Replaced unwrap() chains with ok_or_else() pattern matching
• Fixed git config initialization with proper error fallback
• Added error handling for file_name() operations that could panic on invalid paths
• Used compile-time const evaluation for NonZero values

Before:

if lock.is_none() || lock.as_ref().unwrap().id != req.build_id {
    return Err(...);
}
let active_build = lock.as_ref().unwrap();

After:

let active_build = match lock.as_ref() {
    Some(build) if build.id == req.build_id => build,
    _ => return Err(...),
};

Code Quality

• Fixed typo: suceeded → succeeded
• Added #[must_use] to RateLimit::check()
• Added Default impl for HealthServiceImpl
• Extracted constants: GOFILE_API_BASE, API_STATUS_OK

Documentation

• Module-level docs for git_repo, gofile_api, system_monitor, util, ratelimit, main
• Usage examples for public APIs
• IMPROVEMENTS.md with detailed change summary

Testing

• 8 unit tests for util.rs (path canonicalization, JSON parsing, file discovery)
• 4 unit tests for ratelimit.rs (timing behavior, blocking logic)
• Added tempfile dev dependency for test isolation

✅ Checklist

• I ran cmake + make/ninja and verified the project builds.
• I tested the changes with a live Telegram bot or mock unit tests.
• I updated/add relevant documentation or inline comments.
• I verified that the change does not break existing functionality.
• I followed the project's code style (e.g. clang-format, naming conventions).

🧪 Test Plan

• Applied cargo fmt and verified formatting compliance
• Ran cargo test to validate all unit tests pass
• Code review tool verified no remaining safety issues
• All changes are backward compatible (zero breaking changes)

📝 Notes

Statistics: 11 files, +610/-84 lines, ~200 lines documentation, ~130 lines tests

All error handling improvements maintain existing behavior while eliminating panic points. Tests use descriptive expect() messages for clarity on failure.

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