feat(github): add blob caching with RWMutex

[theakshaypant]

📝 Description of the Change

Implement in-memory blob caching for GitHub provider to reduce duplicate API calls. Cache is thread-safe using RWMutex to allow concurrent reads while ensuring exclusive writes.

Fix tests to use New() factory pattern for proper cache init
Fix ambiguous mutex selector in canIUseCheckrunID()

🔗 Linked GitHub Issue

Fixes #2376

🧪 Testing Strategy

• Unit tests
• Integration tests
• End-to-end tests
• Manual testing
• Not Applicable

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[chmouel]

I think this is going to increase memory more than decrease?

[theakshaypant]

I think this is going to increase memory more than decrease?

This is unrelated to the memory issue but focused on the API optimizations.