fix: don't disable Actor exit_process just because scrapy is importable

[vdusek]

_get_default_exit_process decided the exit_process default from whether import scrapy succeeded, not from whether the Actor is actually running under Scrapy. Any image where Scrapy arrives as a transitive dependency therefore silently disabled the sys.exit() on Actor exit. On the platform, an Actor that signals failure via Actor.fail(exit_code=1) without raising would keep running, end with exit code 0, and get marked SUCCEEDED.

Detection now lives in apify.scrapy._detection and treats the Actor as running under Scrapy when either of these holds:

• run_scrapy_actor is driving the process. It sets an explicit flag before the Actor's main coroutine runs, hence before the Actor initializes.
• The SCRAPY_SETTINGS_MODULE environment variable is set, by the Scrapy CLI or the Actor's entry point. This is the fallback and is already documented as a detection mechanism.

_actor.py consults this detector only through sys.modules, so a non-Scrapy Actor never imports apify.scrapy. Importing it pulls in Scrapy itself (~860 ms), and the old importability check would disable exit_process for exactly such images.

Test environments no longer get exit_process=False implicitly through the scrapy import. The unit _isolate_test_environment fixture now forces the default to False explicitly, since production code must not detect the test environment (see #641). As a side effect, the unit suite no longer needs the scrapy extra installed to avoid SystemExit.

Behavioral change: an Actor that runs Scrapy without using run_scrapy_actor and without setting SCRAPY_SETTINGS_MODULE now defaults exit_process to True. The documented Scrapy setup and the project template always do at least one of those, so legitimate Scrapy Actors are unaffected. I kept this as a plain fix: rather than a breaking change since the old behavior was itself the bug; promote the squash title to fix!: at merge if a major bump is preferred.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.78%. Comparing base (ed1d8eb) to head (af84434).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify/scrapy/_actor_runner.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
+ Coverage   91.75%   91.78%   +0.02%     
==========================================
  Files          50       51       +1     
  Lines        3215     3224       +9     
==========================================
+ Hits         2950     2959       +9     
  Misses        265      265              

Flag	Coverage Δ
e2e	35.45% <16.66%> (-0.20%)	⬇️
integration	57.16% <16.66%> (-0.23%)	⬇️
unit	83.12% <91.66%> (+0.04%)	⬆️

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