Fix native server session lease test

[justinchuby]

Summary

• load the native fixture through AppState so its driver uses the production model-qualified session key
• acquire generation leases through SessionRegistry and close through take_for_close
• assert leases are released after generation and session close

Tests

• cargo check -p onnx-genai-server --features native-backend --all-targets
• cargo test -p onnx-genai-server --features native-backend --all-targets
• cargo test -p onnx-genai-server --lib
• cargo fmt --all -- --check
• cargo clippy -p onnx-genai-server --features native-backend --all-targets -- -D warnings

[justinchuby]

Closing as a duplicate after auditing #2114 and #2115. #2115 correctly accounts for the post-Finished lease-release race and is the implementation being validated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.70%. Comparing base (c00aeb9) to head (bac8f4d).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2118      +/-   ##
==========================================
+ Coverage   80.51%   80.70%   +0.19%     
==========================================
  Files         414      429      +15     
  Lines      200017   215150   +15133     
  Branches   200017   215150   +15133     
==========================================
+ Hits       161037   173638   +12601     
- Misses      33462    35741    +2279     
- Partials     5518     5771     +253     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (?)
cli-ort-windows	72.10% <ø> (?)
mlas	85.90% <ø> (?)
offline	80.83% <ø> (+0.31%)	⬆️

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see 73 files with indirect coverage changes

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