Replace magic number in _FakeDoneEvent with named constant

[Copilot]

The pre_wait class attribute on _FakeDoneEvent used a bare 0.02 float with no explanation of why that specific duration was chosen, making the intent opaque.

Changes

• New constant WORKER_THREAD_STARTUP_DELAY: float = 0.02 defined at module level in tests/test_plugin_xmpp.py, with a comment explaining it exists to give the worker thread enough time to reach a known checkpoint before the test forces the timeout branch — without adding unnecessary wall-clock overhead to the test suite.
• _FakeDoneEvent.pre_wait now references WORKER_THREAD_STARTUP_DELAY instead of the inline literal.

# Before
pre_wait: float = 0.02

# After
# Seconds to sleep before forcing the timeout branch in _FakeDoneEvent.wait().
# This gives the worker thread enough time to progress to a known checkpoint
# (e.g. event loop created, client instantiated) without relying on a longer
# wall-clock timeout that would slow tests down.
WORKER_THREAD_STARTUP_DELAY: float = 0.02

class _FakeDoneEvent:
    pre_wait: float = WORKER_THREAD_STARTUP_DELAY

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