Fix _FakeDoneEvent.signal_evt class-state leakage between XMPP tests

[Copilot]

_FakeDoneEvent.signal_evt was set via direct class attribute assignment in three timeout-path tests, leaving stale state that could affect subsequent tests depending on execution order.

Changes

• Replace all three _FakeDoneEvent.signal_evt = ... direct assignments with monkeypatch.setattr() so pytest's monkeypatch fixture automatically restores the attribute to its original None after each test

# Before — class state persists after test completes or fails
_FakeDoneEvent.signal_evt = client_created

# After — automatically restored by monkeypatch teardown
monkeypatch.setattr(_FakeDoneEvent, "signal_evt", client_created)

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