Log broadcast listener failures

[JjCyberz]

Fixes #716.

When the server statistics broadcast listener task exits, OPAL currently only triggers a worker shutdown. If the task failed because the broadcast URI host cannot be resolved, the useful exception stays buried in dependency DEBUG output and no OPAL server ERROR log points at the broadcast URI.

This PR replaces the anonymous done callback with a named handler that:

• reads the completed broadcast listener task result,
• logs an ERROR when the task has an exception,
• includes the configured broadcast URI and exception repr in the log message,
• still triggers the same graceful worker shutdown behavior afterward.

Validation:

• git diff --check
• python -m py_compile packages/opal-server/opal_server/server.py

Bounty note:

/claim #716

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[JjCyberz]

Added a small unit test file covering the broadcast listener done-callback behavior:

• logs the broadcast URI and exception when the listener task failed
• logs cancellation separately
• still triggers the existing graceful shutdown path

Local verification run:

python -m py_compile packages/opal-server/opal_server/server.py packages/opal-server/opal_server/tests/test_server_broadcast_listener.py

I could not run the pytest target locally because this environment does not have pytest installed, but the test is isolated and uses monkeypatching only around the imported module logger and shutdown method.