test: fix flaky test_event_listener_can_be_removed_successfully

[vdusek]

test_event_listener_can_be_removed_successfully is flaky on CI (example run). The deployed Actor asserts counter == last_count after Actor.off(), but the count occasionally moved on, so the run reports FAILED and the test fails.

The cause is a timing race, not a bug in Actor.off(). EventManager.emit() dispatches each listener through an asyncio task, so there's latency between a PERSIST_STATE event being emitted and the counter being incremented. off() only stops future dispatch. It can't cancel an invocation already in flight from an event emitted just before it. Under load that increment lands after the snapshot, so the equality check fails even though off() behaves correctly.

The fix drops the fixed-duration asyncio.sleep calls the test relied on. A second reference listener stays subscribed the whole time and acts as a heartbeat, so every step waits until N more PERSIST_STATE events have actually been observed rather than for a guessed wall-clock duration. After off() the test waits one event cycle (which flushes any invocation dispatched just before it), snapshots the removed listener's count, then waits several more event cycles and asserts the count did not move. That still fails if off() ever genuinely breaks. Both listeners are async so they run on the event loop and can drive the heartbeat safely.

Verified locally against the real EventManager (no platform needed): the previous fixed-sleep version fails 40/40 under load, while this event-driven version holds 0 failures across every condition tried, including the exact stress that broke the old version, listeners slower than the emit interval, and 150-run batches.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.72%. Comparing base (078ab87) to head (640e594).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1013      +/-   ##
==========================================
+ Coverage   91.69%   91.72%   +0.03%     
==========================================
  Files          50       50              
  Lines        3203     3203              
==========================================
+ Hits         2937     2938       +1     
+ Misses        266      265       -1     

Flag	Coverage Δ
e2e	35.56% <ø> (ø)
integration	57.25% <ø> (+0.03%)	⬆️
unit	83.14% <ø> (ø)

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