Improve Gemini error handling and document fallback behaviorYoutube relevance finder

[asabo-dev]

Summary

This PR enhances the YouTube Relevance Finder by improving error handling around
Gemini API usage and documenting fallback behavior when API limits or failures occur.

The goal is to make the application more robust and suitable for learning and
experimentation under real-world API constraints.

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What Changed

• Added graceful fallback logic when Gemini API calls fail or return unexpected responses
• Improved error handling with clear, user-friendly warning messages
• Refactored logic into clearer, responsibility-focused classes
• Updated README to document:
  • Differences from the original implementation
  • Known API limitations
  • Intentional fallback behavior

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Why This Change

Gemini API usage can be affected by quota limits, availability, or malformed responses.
Instead of allowing the application to fail, this implementation ensures the script
continues running by assigning a default relevance score when necessary.

This reflects real-world defensive programming practices and improves the developer
learning experience.

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Testing

• Manually tested with valid API keys
• Verified fallback behavior when Gemini API calls fail or exceed quota
• Confirmed script continues execution without crashing

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Notes

• No breaking changes to the original interface
• Fallback behavior is intentional and documented
• Changes are scoped to error handling, structure, and documentation only

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