Add validate tirgger events in dbt cloud deferrable tasks

[takayoshi-makabe]

DbtCloudRunJobOperator.execute_complete and DbtCloudJobRunSensor.execute_complete used the trigger event without validating it:

• event=None: happens when a deferred task resumes without a payload — a custom or subclassed trigger that yields TriggerEvent(None), or an event payload lost between the triggerer and the worker. Both methods crashed with an opaque TypeError: 'NoneType' object is not subscriptable.
• Unrecognised status: happens on triggerer/worker version skew (a task defers under one provider version and resumes under another whose trigger emits a different status vocabulary) or with a custom trigger. DbtCloudRunJobTrigger emits success/cancelled/error/timeout, but both execute_complete methods branch deny-list style (only specific failure statuses raise, everything else succeeds), so an unknown status fell through into the success path — the task was marked SUCCESS even though the dbt Cloud job outcome was unknown.
• The sensor additionally never checked timeout at all — it only raised on error/cancelled, so a job run that timed out while deferred was silently marked SUCCESS.

This adds a validate_execute_complete_event() helper (with a dedicated DbtCloudTriggerEventException), applied at the top of both execute_complete methods. Same fix shape as the anthropic (#69379) and OpenAI (#69506) providers: the None check mirrors the amazon provider's helper of the same name, and the status check plays the role amazon's call sites cover with their if validated_event["status"] != "success": raise allow-list branching.

The sensor's fix also replaces a bare raise AirflowException() (no message) with DbtCloudJobRunException(event["message"]), which both fixes the missing timeout handling and surfaces the trigger's actual message instead of an empty exception.

The operator keeps its existing per-status branching (cancelled/error/timeout each have distinct messages, and timeout additionally attempts job cancellation) since validate_execute_complete_event() already closes the status set to the four values above.

• related: Validate trigger events in Anthropic deferrable tasks #69379

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