Fix: Add authentication to pull request diff retrieval for private repositories

[github-actions]

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Fixed an issue with the get_diff_of_pull_request function by adding GitHub authentication headers to the requests.get() call. The function was failing to retrieve diffs from private repositories because it lacked proper authentication. Now, it uses the GitHub API token from environment variables to create an authorization header, allowing access to both public and private repository diffs.

• src/git_bob/_github_utilities.py Added authentication headers to requests.get() call to allow access to private repository diffs.

closes #726

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.16%. Comparing base (998fe95) to head (fc24289).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #727      +/-   ##
==========================================
+ Coverage   59.11%   59.16%   +0.04%     
==========================================
  Files          14       14              
  Lines        1886     1888       +2     
==========================================
+ Hits         1115     1117       +2     
  Misses        771      771              

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