fix: Reuse created GitHub client per each installation

[konradasb]

Description

Related Issue(s)

Checklist

• I have read the contribution guidelines.
• I have signed off my commits.
• I have added necessary documentation (if appropriate).
• I have added tests to cover my changes.

Summary by CodeRabbit

Release Notes

• Improvements
  • Optimized GitHub client performance through improved caching across multiple installations.
  • Enhanced system event visibility with updated logging levels for pool operations and VM lifecycle events.

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[coderabbitai]

Caution

Review failed

The pull request is closed.

📝 Walkthrough

Walkthrough

Two files are modified: helper/github/client.go introduces thread-safe, per-installation caching for GitHub clients using a mutex and map to store and reuse clients by installation ID, while server/pool.go elevates logging visibility by promoting several Debug-level messages to Info level.

Changes

Cohort / File(s)	Summary
GitHub Client Caching helper/github/client.go	Adds sync.Mutex and installations map[int64]*github.Client fields to support thread-safe, per-installation client caching. The Installation() method now checks the cache under lock, reuses existing clients, or creates and stores new ones instead of returning a fresh client each time.
Logging Level Updates server/pool.go	Switches four log messages from Debug to Info level: pool scaling finalization, VM start success, VM exit/cleanup, and scale-down confirmation. Message text is also updated. No functional behavior changes beyond increased log visibility.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

✨ Finishing touches

• 📝 Generate docstrings

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