test: mark test_get_snapshot with run_alone to deflake CI

[vdusek]

test_get_snapshot failed on macos-latest / Python 3.10 in run 31388378733 on assert snapshot.screenshot is not None, with Chromium answering Protocol error (Page.captureScreenshot): Unable to capture screenshot.

The macOS image is the smallest of the three (~3 vCPU / 7 GB), yet the -m "not run_alone" pass drives 8 xdist workers. At the failure timestamp three of them were each running their own Chromium: gw7 in test_get_snapshot, gw0 cycling the test_chromium_headless_headers parametrizations, and gw4 in the stagehand browser-controller tests. Under that contention the renderer cannot composite a frame for a full_page capture, and Playwright does not retry the error.

PlaywrightPreNavCrawlingContext.get_snapshot() deliberately degrades that failure to screenshot=None - an error snapshot must never break a crawl, and ErrorSnapshotter already guards on it - so the test observes None. The nondeterminism is runner contention, not a race in Crawlee, so product code is unchanged.

@pytest.mark.run_alone moves the test into the serial --numprocesses=1 pass, removing the contention while keeping every assertion intact. This is the second tier of the hierarchy documented in tests/unit/README.md; the same file already marks test_firefox_headless_headers and test_isolation_cookies this way.

The failure does not reproduce on Linux - 0/20 runs of the Playwright module at -n 16 (oversubscribing 8 cores) - so macOS CI is what re-verifies the fix. Locally the marker routes the test correctly (53 collected in the parallel pass, 4 in the serial one) and the full run_alone group is green at 14 items.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.57%. Comparing base (165223e) to head (a300a7d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2150      +/-   ##
==========================================
+ Coverage   93.55%   93.57%   +0.01%     
==========================================
  Files         181      181              
  Lines       12648    12648              
==========================================
+ Hits        11833    11835       +2     
+ Misses        815      813       -2     

Flag	Coverage Δ
unit	93.57% <ø> (+0.01%)	⬆️

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