Increased test coverage in crates/common and crates/types

[GAlexeyV]

Issue to address: #184

Summary

Added tests across 11 modules.

Coverage (Measured by cargo llvm-cov)

helix-common

• Overall: 67.95% lines covered (3,132 / 4,612 lines)
• 100% coverage: api_provider.rs, builder_info.rs, proposer.rs, signing.rs
• 99%+ coverage: validator.rs (99.26%)
• 90%+ coverage: chain_info.rs (92.74%)
• 80%+ coverage: local_cache.rs (76.49%), utils.rs (69.55%)
• 0% coverage: alerts.rs, data_api.rs, metrics.rs, task.rs, traces (not tested in lib tests) - could be a next step

helix-types

• Overall: 64.02% lines covered (1,759 / 2,746 lines)
• 100% coverage: error.rs, fields.rs, clock.rs
• 90%+ coverage: spec.rs (91.49%), builder_bid.rs (95.65%)
• 50-90% coverage: blobs.rs (42.49%), execution_payload.rs (39.86%), bid_submission.rs (35.82%)
• 0% coverage: hydration.rs, validator.rs, lib.rs (not tested in lib tests) - could be a next step

Combined

• Total: 67.95% overall coverage
• 217 tests passing (common: 131, types: 86)
• 0 warnings

Potential Bug Discovery
Holesky network starts at Bellatrix fork (post-merge), not Base fork like Mainnet/Sepolia.

Next Steps - Tests to Improve

• bid_submission.rs (35.82% coverage)
• execution_payload.rs (39.86% coverage)
• blobs.rs (42.49% coverage)
• config.rs (30.10% coverage)

Integration tests - more complex