Implement GitHub App authentication with fallback

[hanna-skryl]

This PR implements GitHub App installation authentication for the Code PushUp GitHub Action with automatic fallback to standard token authentication.

• Add an authenticate() function that retrieves installation tokens from the Cloud Run service
• Fall back to the standard token when the app is unavailable
• Add unit tests with fetch mocking
• Update main.ts to use the new authentication flow

[github-actions]

Code PushUp

🥳 Code PushUp report has improved – compared current commit 5bb3fdd with previous commit 5b22a49.

🕵️ See full comparison in Code PushUp portal 🔍

🏷️ Categories

🏷️ Category	⭐ Previous score	⭐ Current score	🔄 Score change
Code coverage	🟡 59	🟡 62
Bug prevention	🟢 100	🟢 100	–
Code style	🟢 100	🟢 100	–
Security	🔴 15	🔴 15	–
Updates	🟢 94	🟢 94	–

👍 1 group improved, 👍 3 audits improved

🗃️ Groups

🔌 Plugin	🗃️ Group	⭐ Previous score	⭐ Current score	🔄 Score change
Code coverage	Code coverage metrics	🟡 59	🟡 62

11 other groups are unchanged.

🛡️ Audits

🔌 Plugin	🛡️ Audit	📏 Previous value	📏 Current value	🔄 Value change
Code coverage	Branch coverage	🟥 38 %	🟥 42.9 %
Code coverage	Line coverage	🟨 57.5 %	🟨 60.4 %
Code coverage	Function coverage	🟨 69 %	🟨 71.9 %

169 other audits are unchanged.

[matejchalk]

Well done 👏