Fix AwaitMessageTrigger missing super().__init__() call

[david-parkk]

Summary

Fix AwaitMessageTrigger not calling super().__init__(), which caused an AttributeError
when the trigger was used as an AssetWatcher trigger.

Analysis

BaseTrigger.__init__() initialises self._task_instance = None, which is required for the
task_instance property to work correctly:

# airflow-core/src/airflow/triggers/base.py
class BaseTrigger:
    def __init__(self, **kwargs):
        self._task_instance = None  # ← must be set by super().__init__()
        ...

    @property
    def task_instance(self):
        return self._task_instance  # ← AttributeError if _task_instance never set

AwaitMessageTrigger.__init__() was missing the super().__init__() call, so
_task_instance was never initialised.

When the trigger is used as an AssetWatcher trigger, there is no associated task instance,
so _task_instance is never set. When the triggerer later accesses trigger.task_instance,
it raises:

AttributeError: 'AwaitMessageTrigger' object has no attribute '_task_instance'

This was confirmed by running KafkaMessageQueueTrigger (which delegates to
AwaitMessageTrigger) as an AssetWatcher trigger in a Breeze dev environment.

Changes

Added super().__init__() call to AwaitMessageTrigger.__init__().

Test

• Added test_trigger_has_task_instance_attribute — verifies that _task_instance is
  properly initialised to None after instantiation, which is the condition required for
  AssetWatcher usage to work without AttributeError
• All provider unit tests pass (uv run --project providers/apache/kafka pytest providers/apache/kafka/tests/unit/)
• Static checks pass (prek run --from-ref main --stage pre-commit)

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I'm happy to make any adjustments based on your feedback. Thank you to the maintainers for taking the time to review this contribution!

[david-parkk]

@vincbeck Hi, thank you for reviewing and approving my PR!

I noticed that a similar change has already been merged by another member (#66302), so I’ll go ahead and close this PR.

I really appreciate your time. Looking forward to contributing more in the future!