JobStartDateDeadline

[Ashish0253]

Description
This PR implements the JobStartDateDeadline as part of AIP-86 (Deadline Alerts). This new reference type allows users to set deadline alerts based on the start time of background Jobs (like the Scheduler or Triggerer), separating system-level monitoring from DAG-level logic.

Closes: #44003

Changes
Core Models: Added JobStartDateDeadline to airflow/models/deadline.py and integrated it into the ReferenceModels registry.

Task SDK: - Exposed JOB_START_DATE in airflow/sdk/definitions/deadline.py for user-facing DAG definitions.

Created a new JOB category in DeadlineReference.TYPES and updated the ALL registry to include it.

Testing: Updated REFERENCE_TYPES in the Task SDK test suite and added equality/hash validation for the new Job reference.

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