test: Fix DeprecationWarning and RuntimeWarning emitted by unit tests

[vdusek]

Description

The unit-test jobs in CI emit two kinds of warnings:

• DeprecationWarning: This process is multi-threaded, use of fork() may lead to deadlocks in the child from multiprocessing on Linux with Python 3.12+. It is triggered by test_system.py (explicit fork context), the ProcessPoolExecutor tests in test_basic_crawler.py, and the proxy.py acceptor processes used by the proxy fixtures, because the default start method on Linux up to Python 3.13 is fork and the pytest process is multi-threaded (uvicorn test server thread, xdist).
• RuntimeWarning: coroutine 'AsyncMockMixin._execute_mock_call' was never awaited in test_global_and_local_event_manager_in_crawler_run, which called an AsyncMock without awaiting it.

Changes:

• Set the spawn start method in tests/unit/conftest.py for all subprocesses created in unit tests. This also aligns Python <= 3.13 runs with Python 3.14, which no longer defaults to fork. proxy.py has no API for choosing a start method, so the default has to be set globally.
• Refactor the shared-memory estimation test in test_system.py to module-level (picklable) child functions running under an explicit spawn context instead of fork.
• Replace the unawaited AsyncMock with a plain Mock, since the handler only records call arguments synchronously.

The full unit suite passes with an empty warnings summary.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.95%. Comparing base (3fd5ace) to head (6fc81cd).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1960      +/-   ##
==========================================
- Coverage   93.00%   92.95%   -0.06%     
==========================================
  Files         167      167              
  Lines       11737    11737              
==========================================
- Hits        10916    10910       -6     
- Misses        821      827       +6     

Flag	Coverage Δ
unit	92.95% <ø> (-0.06%)	⬇️

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