Achieve 100% test coverage

[samuel-williams-shopify]

Summary

• add focused tests to achieve complete code coverage
• reject unsupported compression encodings instead of emitting invalid compressed frames
• report unsupported inbound compression as UNIMPLEMENTED
• reject truncated gRPC prefixes and payloads while preserving clean end-of-stream handling
• rely on the gRPC header policy for structured status and message extraction

Verification

• COVERAGE=PartialSummary bundle exec bake test — 225 tests, 400 assertions, 492/492 lines (100.0%)
• bundle exec bake covered:validate --paths .covered.db \;
• bundle exec rubocop