test(fuzz): make client fixture module-scoped to prevent startup failures

[callebtc]

Fixes fuzz test failures where alternating tests failed during TestClient(app) setup with SystemExit(1) / CancelledError.

The function-scoped client fixture was starting and shutting down the global FastAPI app for every fuzz test. After the first teardown, the next startup failed (likely due to the management RPC server lifecycle / global state not being fully reset). Making the fixture module-scoped starts the app once for the entire fuzz module and avoids the lifecycle conflict.

Verified with make test:

720 passed, 67 skipped, 33 warnings in 419.90s

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.29%. Comparing base (066a771) to head (2c97388).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1051      +/-   ##
==========================================
- Coverage   74.32%   74.29%   -0.03%     
==========================================
  Files         111      111              
  Lines       12372    12372              
==========================================
- Hits         9195     9192       -3     
- Misses       3177     3180       +3     

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