fix(python): Return None from get() on 404 to match Rust client

[jan-auer]

Session.get() now returns None when the object does not exist, matching
the behavior of head() and the Rust client. Previously it raised
RequestError with status 404, forcing callers to catch and inspect the
exception for the common not-found case.

This is a breaking change for callers that catch RequestError to detect
missing objects — they should check for None instead.

Fixes FS-427

[linear-code]

FS-427

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.66%. Comparing base (e88b7f8) to head (4bc81f4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
- Coverage   87.67%   87.66%   -0.02%     
==========================================
  Files          93       93              
  Lines       15113    15116       +3     
==========================================
  Hits        13251    13251              
- Misses       1862     1865       +3     

Components	Coverage Δ
Rust Backend	92.45% <ø> (ø)
Rust Client	79.89% <ø> (ø)
Python Client	90.25% <100.00%> (+0.04%)	⬆️

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[cursor]

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