Improve test coverage

[samuel-williams-shopify]

Summary

• cover context-section lookup stopping at the next top-level heading
• cover malformed installer index fallback behavior
• cover generated metadata for Markdown without a heading and with multiple paragraphs

Testing

• COVERAGE=PartialSummary bundle exec bake test (32 tests, 80 assertions, 100% coverage)
• bundle exec bake covered:validate
• bundle exec bake decode:index:coverage lib
• Ruby syntax checks for both modified test files

Note

• bundle exec rubocop is currently blocked before file inspection because the repository configuration references unsupported Layout/BlockDelimiterSpacing and Style/GlobalExceptionVariables cops.