docs(spec): update CODE_REVIEW_ISSUES.md with complete implementation status

Document all 3 phases of code review fix implementation:
- Phase 1: Critical fixes (Issue #1, #2) - cbd768b
- Phase 2: Code quality (Issue #3) - e6fdfb3
- Phase 3: Testing & validation - 19beda1

All issues resolved:
- ✅ Issue #1 (CRITICAL): AttributeError fixed
- ✅ Issue #2 (HIGH): Duplicate method removed
- ✅ Issue #3 (MEDIUM): Documentation updated

Test results: 6/6 unit tests passing
Status: Ready for PR re-review

Spec: kafka-protobuf-binance-e2e
PR: #16 (feature/kafka-proto-backend)

Author: tommy-ca