fix: reconnect to platform events websocket after connection drop

[vdusek]

Description

• Previously, ApifyEventManager._process_platform_messages connected to the platform events websocket exactly once. Any dropped connection raised ConnectionClosedError and permanently ended the processing task, while a graceful server-side close exited silently. In both cases, the Actor missed all subsequent platform events (MIGRATING, ABORTING, PERSIST_STATE) for the rest of the run.
• The connection now uses the websockets reconnect iterator, which re-establishes the connection after every drop or graceful close, with backoff on failed attempts. An abnormal drop is logged as a warning and a graceful close as info, both with the close code and reason, and a successful reconnect is logged as well.
• A process_exception callback keeps errors fatal before the first successful connection, so Actor.init still fails fast on a misconfigured URL instead of hanging. After the first connection, the default websockets classification decides which errors are transient and retried.
• __aexit__ now cancels the processing task before closing the websocket. Otherwise, every clean shutdown would trigger a spurious reconnect attempt.
• Added a regression test, parametrized over graceful and abnormal closes, that drops the connection server-side and asserts the drop is logged, the client reconnects, and events keep arriving. It replaces the obsolete mid-stream disconnect test, whose premise (the task ends after a drop) no longer holds.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.98%. Comparing base (2cc5602) to head (fe082f1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/apify/events/_apify_event_manager.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #967      +/-   ##
==========================================
+ Coverage   89.91%   89.98%   +0.06%     
==========================================
  Files          49       49              
  Lines        3085     3095      +10     
==========================================
+ Hits         2774     2785      +11     
+ Misses        311      310       -1     

Flag	Coverage Δ
e2e	35.92% <7.14%> (-0.06%)	⬇️
integration	56.83% <7.14%> (-0.16%)	⬇️
unit	78.83% <96.42%> (+0.13%)	⬆️

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