Update ZonalFile write operations with byte count and location tracking

[suni72]

Description

Updates ZonalFile.write() to properly return the number of bytes written and track the logical file position (self.loc).

Why: Missing byte returns and static file pointers caused downstream tools like PyTorch to calculate 0-length chunks, corrupting saved checkpoint metadata.

Changes

• ZonalFile.write(): Now returns len(data) and increments self.loc += len(data).
• Tests: Updated test_zonal_file_write_success to assert correct byte returns and sequential loc accumulation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.45%. Comparing base (1533f79) to head (bba8a2b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #777      +/-   ##
==========================================
+ Coverage   75.08%   75.45%   +0.37%     
==========================================
  Files          14       14              
  Lines        2625     2628       +3     
==========================================
+ Hits         1971     1983      +12     
+ Misses        654      645       -9     

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[ankitaluthra1]

/gcbrun