fix: coerce null stats in Apify request queue metadata

[vdusek]

Opening a request queue crashed when the Apify API omitted stats (sending an explicit null): model_dump(by_alias=True) preserved stats: None, and the non-optional stats field on ApifyRequestQueueMetadata rejected it. The default_factory only fires when the key is absent, not when it is present-but-null.

This adds a BeforeValidator that coerces None to a default RequestQueueStats, mirroring the defensive metadata.stats or RequestQueueStats() already used in get_metadata(), so every validation path is protected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.88%. Comparing base (2cc5602) to head (a590ff9).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #974      +/-   ##
==========================================
- Coverage   89.91%   89.88%   -0.04%     
==========================================
  Files          49       49              
  Lines        3085     3085              
==========================================
- Hits         2774     2773       -1     
- Misses        311      312       +1     

Flag	Coverage Δ
e2e	35.98% <100.00%> (ø)
integration	56.95% <100.00%> (-0.04%)	⬇️
unit	78.70% <100.00%> (ø)

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