Add validate trigger events deferrable tasks OpenAI provider

[takayoshi-makabe]

OpenAITriggerBatchOperator.execute_complete used the trigger event without validating it, branching deny-list style (only status == "error" raised, everything else succeeded):

• cancelled was marked SUCCESS: the provider's own OpenAIBatchTrigger emits {"status": "cancelled"} when the batch is cancelled while the task is deferred, but only error raised — so the task went green with no results. The non-deferrable path (OpenAIHook.wait_for_batch) has raised for CANCELLED/CANCELLING since feat(providers/openai): support batch api in hook/operator/trigger #41554, so the two modes of the same operator disagreed.
• event=None: crashed with an opaque TypeError: 'NoneType' object is not subscriptable.
• Unrecognised status (triggerer/worker version skew, custom trigger): fell through into the success path.

Same fix shape as the anthropic provider in #69379: a validate_execute_complete_event() helper plus allow-list branching (status != "success" raises).

Also fixes a missing else in OpenAIBatchTrigger.run() that yielded a second, spurious error event after every terminal event (benign — only the first event is consumed — but incorrect).

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