feat: [PubSub] Add MessageTransformationFailureReason to IngestionFailureEvent (#8403)

* feat: Add MessageTransformationFailureReason to IngestionFailureEvent
docs: Document that the `acknowledge_confirmation` and `modify_ack_deadline_confirmation` fields in message `.google.pubsub.v1.StreamingPullResponse` are not guaranteed to be populated

PiperOrigin-RevId: 776198381

Source-Link: googleapis/googleapis@ade5fe4

Source-Link: googleapis/googleapis-gen@2e304e4
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

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Author: gcf-owl-bot[bot]