🧪 test: add unit tests for custom parseMetadataYaml function

[akutuva21]

🎯 What: The testing gap addressed
This PR addresses the lack of tests for the parseMetadataYaml custom parser function located in scripts/validate-metadata.js. The function's main() runner was modified so that the script acts as a library when imported and as an executable script only when run directly, to allow for unit tests.

📊 Coverage: What scenarios are now tested
Tests have been introduced using built-in node:test covering:

• Basic key-value pairs parsing
• Ignoring comments and empty lines
• Proper list parsing and creation
• Properly nesting objects
• Properly nesting deeply nested objects
• Safely handling an empty tags array

✨ Result: The improvement in test coverage
Tests are now fully working and deterministic using the built-in Node.js testing module and node:assert, giving more confidence for potential refactoring in the future.

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PR created automatically by Jules for task 7716385411393383498 started by @akutuva21

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[akutuva21]

Reviewed in sequence. parseMetadataYaml tests are solid; branch now exports parser/validator helpers additively for compatibility.