Fix: add safe decode_file fallback for “encoding: none” and refactor callers to use it.

[github-actions]

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This PR fixes Issue #819 by handling GitHub’s “encoding: none” responses that caused PyGithub’s decoded_content assertion to fail. I added a robust decode_file() that first tries the standard path and, on assertion failure, falls back to downloading via download_url or the Git blob API, preserving the existing behavior of raising UnicodeDecodeError for binary content. I then refactored internal callers to use decode_file() instead of directly accessing decoded_content.decode() in get_repository_file_contents, rename_file_in_repository, and copy_file_in_repository.

• src/git_bob/_github_utilities.py Implemented a fallback-aware decode_file() and updated all direct decoded_content.decode() usages to call it.

closes #819

[codecov]

Codecov Report

❌ Patch coverage is 12.50000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.08%. Comparing base (186eff5) to head (cd65780).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/git_bob/_github_utilities.py	12.50%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #820      +/-   ##
==========================================
- Coverage   58.58%   58.08%   -0.51%     
==========================================
  Files          14       14              
  Lines        1910     1930      +20     
==========================================
+ Hits         1119     1121       +2     
- Misses        791      809      +18     

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