fix(gcs): Set content length on empty resumable requests

[jan-auer]

GCS rejects resumable upload offset queries without Content-Length: 0 with HTTP 411. This prevents tiered storage from writing the final chunk.

Offset queries and cancellation requests now explicitly send Content-Length: 0, matching Google’s documentation for status queries and cancellation. The emulator does not enforce this requirement, which is why this is not detected in tests.

Fixes OBJECTSTORE-41P

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.34%. Comparing base (25644d5) to head (cc5781e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
- Coverage   91.34%   91.34%   -0.01%     
==========================================
  Files         117      117              
  Lines       24171    24173       +2     
==========================================
+ Hits        22080    22081       +1     
- Misses       2091     2092       +1     

Components	Coverage Δ
Rust Backend	94.82% <100.00%> (+<0.01%)	⬆️
Rust Client	81.70% <ø> (ø)
Python Client	93.81% <ø> (ø)

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