fix: Improve GitHub API rate limit handling with PAT support

[noonowl]

Description

This PR improves the GitHub API rate limit handling in the Kotlin Language Server extension by adding proper Personal Access Token (PAT) support.

Changes

• Fix GitHub API authentication header format to use Authorization: Bearer <token>
• Add conditional token header addition
• Add helpful warning message when rate limit is exceeded without a token

Why

GitHub's API has rate limits for unauthenticated requests. When users hit these limits, they get 403 errors. By supporting Personal Access Tokens (PATs), users can increase their rate limits and avoid these errors.

How to use

1. Create a GitHub Personal Access Token with repo scope
2. Set the token as an environment variable:

   # macOS/Linux
   export GITHUB_TOKEN=your_token_here
   
   # Windows (PowerShell)
   $env:GITHUB_TOKEN="your_token_here"