Fix pull request diff retrieval for private repositories using GitHub's REST API

[github-actions]

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I modified the get_diff_of_pull_request function to properly handle diffs from private repositories by using GitHub's REST API endpoint with the correct Accept header. Instead of using the diff_url property which returns a web UI URL, the function now constructs the proper API endpoint (https://api.github.com/repos/{owner}/{repo}/pulls/{number}) and includes both the Authorization token and the required Accept header (application/vnd.github.v3.diff) to specifically request diff content. This change ensures that the function can retrieve diffs from both public and private repositories when properly authenticated.

• src/git_bob/_github_utilities.py Updated pull request diff retrieval to use GitHub's REST API endpoint with proper Accept header for private repository access.

closes #726

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.05%. Comparing base (c4145ae) to head (a2f434b).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #745   +/-   ##
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  Coverage   59.05%   59.05%           
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  Files          14       14           
  Lines        1893     1893           
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  Hits         1118     1118           
  Misses        775      775           

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