Achieve complete test coverage

[samuel-williams-shopify]

Summary

• add coverage for client construction, inheritance, representations, and invalid streaming modes
• cover dispatcher service-name mismatch and missing-handler responses
• make blockless bidirectional calls close their request stream and preserve the returned response stream
• reach 100% executable line coverage

Validation

• COVERAGE=BriefSummary,PartialSummary bundle exec sus test (256/256 lines, 100%)
• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• git diff --check