Validate deferred trigger classpath resolves to a BaseTrigger subclass

[hypnguyen1209]

The triggerer resolves a trigger from the classpath carried in the deferred-task Execution API payload via import_string(classpath) and then instantiates it with trigger_class(**kwargs). get_trigger_by_classpath did not check that the imported object is actually a BaseTrigger subclass, so any importable callable (e.g. subprocess.check_output) could be loaded and invoked in the long-running triggerer process.

Harden this by rejecting, before caching and before instantiation, any classpath that does not resolve to a type that is a BaseTrigger subclass. Legitimate triggers are unaffected (they are always BaseTrigger subclasses); an invalid classpath now fails the trigger cleanly instead of being instantiated. Adds a unit test covering both the accepted and rejected cases.

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[Copilot]

Pull request overview

Hardens the triggerer’s deferred-trigger loading path by validating that a deferred-task classpath resolves to a BaseTrigger subclass before caching/instantiation, preventing arbitrary importable callables from being invoked in the triggerer process.

Changes:

• Add a BaseTrigger-subclass validation (and clearer docstring) in TriggerRunner.get_trigger_by_classpath.
• Add a unit test covering both an allowed trigger class and a rejected non-trigger callable.
• Add a release-note newsfragment describing the security hardening (but the fragment filename needs correction).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
airflow-core/src/airflow/jobs/triggerer_job_runner.py	Validates imported objects are BaseTrigger subclasses before caching/instantiation.
airflow-core/tests/unit/jobs/test_triggerer_job.py	Adds regression test for accepting a real trigger and rejecting a non-trigger callable classpath.
airflow-core/newsfragments/99999.bugfix.rst	Adds release note entry for the hardening (currently named with a placeholder).