beacon/blsync: Add comprehensive finality test coverage

[floor-licker]

Summary

Adds comprehensive test coverage for beacon block sync finality handling in response to the TODO comment noting the gap in coverage.

Changes Made

• New Test Function: TestBlockSyncFinality() with comprehensive finality scenarios
• Enhanced Test Infrastructure: Updated testHeadTracker to support proper finality testing
• Improved Documentation: Added detailed comments explaining beacon chain finality concepts

Test Scenarios Covered

1. Basic finality with proper finality update - Tests normal case with attested and finalized blocks
2. No finality update available - Tests fallback behavior when no finality data exists
3. Direct ValidatedFinality method testing - Tests the finality method directly

Testing

• All existing tests continue to pass
• New test passes consistently
• Verified with go test ./beacon/blsync -v

[floor-licker]

Hey @zsfelfoldi - just wanted to check if you had a chance to review this.