Add failure-path and serialization tests for StepFunctionsExecutionCompleteTrigger

[bingqin2]

The region_name fix originally proposed here was merged in the meantime via #72625, so this PR is re-scoped to the parts that PR did not cover:

• test_run_failure: the trigger's error path, i.e. async_wait raising AirflowException yields a TriggerEvent with status="error", the message, and the execution ARN
• test_serialization_with_verify_and_botocore_config: verify and botocore_config round-trip through serialize()
• test_serialization_omits_unset_hook_params: region_name, verify, and botocore_config are pruned from the serialized kwargs when not set; before Fix region_name being ignored by the Step Functions execution trigger #72625 the trigger always serialized region_name, even as None
• documents the region_name parameter in the trigger docstring

The tests merged in #72625 are left unchanged; these are additive.

related: #72625

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[boring-cyborg]

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[bingqin2]

Thanks for the approval @vincbeck. Since the same fix landed via #72625 while this one was waiting, I have rebased and re-scoped this PR to what is still missing after that merge: the trigger's failure-path test, serialization tests for verify/botocore_config and for the pruning of unset hook parameters, and the region_name docstring entry. The tests from #72625 are untouched. CI is re-running now.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.