fix: prevent duplicate platform websocket when nesting ApifyEventManager

[vdusek]

ApifyEventManager overrode Crawlee's EventManager async context manager without respecting the parent's ref counting. Crawlee ref-counts nested async with blocks, and BasicCrawler._run_crawler always re-enters the same global event manager that async with Actor: already entered. Because the override started the platform websocket machinery on every enter and tore it down on every exit, the standard async with Actor: + await crawler.run() pattern opened a second websocket connection.

On the platform this meant every platform event (MIGRATING, ABORTING, PERSIST_STATE) was delivered twice during a crawl, and on the inner exit the fields pointed at the second connection, so the first leaked and could never be cancelled. Each sequential crawler.run() added another connection; the platform caps at 10 per run and then closes with 1008 (a non-retryable code), permanently disabling platform events - so migration/abort handling silently stops for the rest of the run.

The fix mirrors the parent's ref counting: the websocket machinery is started only on the outermost enter (0 -> 1) and torn down only on the outermost exit (1 -> 0). Nested enters/exits reuse the single existing connection.

Added two regression tests: a nested enter reuses the one connection and delivers each event exactly once; a nested exit leaves the outer context's connection working while the final exit tears everything down with no leaked task.

✍️ Drafted by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.84%. Comparing base (58d6da1) to head (fbfec76).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1059      +/-   ##
==========================================
- Coverage   91.89%   91.84%   -0.05%     
==========================================
  Files          51       51              
  Lines        3232     3239       +7     
==========================================
+ Hits         2970     2975       +5     
- Misses        262      264       +2     

Flag	Coverage Δ
e2e	35.25% <0.00%> (-0.08%)	⬇️
integration	56.96% <0.00%> (-0.13%)	⬇️
unit	83.23% <100.00%> (-0.03%)	⬇️

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