Fix flaky test: use retry_asserts for observer event propagation

[devin-ai-integration]

Fixes flaky test_consume_events_ignores_non_in_flight_flow_runs (and the similar test_observer_consumes_cancelling_events) in tests/test_observers.py.

Root cause: Both tests emit events via emit_event() and then assert the observer's callback was invoked after a fixed asyncio.sleep(0.2). Events must travel through the EventsWorker → websocket → PrefectEventSubscriber pipeline before reaching the observer's _consume_events loop, and 0.2 seconds is insufficient under CI load.

Fix: Replace the fixed sleep + immediate assertion with the existing retry_asserts utility (from prefect._internal.testing), retrying up to 5 times with a 0.5 s delay between attempts. This is the same pattern used elsewhere in the test suite for eventual-consistency scenarios.

Failed workflow run: https://github.com/PrefectHQ/prefect/actions/runs/23958380669

Checklist

• This pull request references any related issue by including "closes <link to issue>"
  • No existing issue; this is a small flaky-test fix on main.
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

Link to Devin session: https://app.devin.ai/sessions/ed3a43bb4a694ba79c59949174bd7d8c

[devin-ai-integration]

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