test: use spawn context for ProcessPoolExecutor in basic crawler tests

[vdusek]

The state persistence tests in test_basic_crawler.py start ProcessPoolExecutor with the default fork start method. On Linux with Python 3.12 and 3.13, forking a multi-threaded process emits DeprecationWarning: This process is multi-threaded, use of fork() may lead to deadlocks in the child. The executors now pass mp_context=get_context('spawn').

Each spawned worker imports crawlee.crawlers again, which adds a few seconds to each of the two tests. Windows (spawn) and Python 3.14 (forkserver) already pay that cost.

Closes #2273

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.91%. Comparing base (87561ee) to head (edf5b7d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2276   +/-   ##
=======================================
  Coverage   93.90%   93.91%           
=======================================
  Files         182      182           
  Lines       13144    13144           
=======================================
+ Hits        12343    12344    +1     
+ Misses        801      800    -1     

Flag	Coverage Δ
unit	93.91% <ø> (+<0.01%)	⬆️

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[Mantisus]

LGTM

[apify-approval-mirror]

Mirroring approval from @Mantisus.