fix(providers/databricks): DatabricksWorkflowTaskGroup launch task now waits for upstream tasks

[shubhgurav0590]

Closes #51598

Problem

When using DatabricksWorkflowTaskGroup, the internal launch task
ignored upstream task dependencies and executed immediately at DAG
start, regardless of trigger_rule configuration.

Fix

Added create_databricks_workflow_task.set_upstream(root_task) in
the __exit__ method so the launch task correctly waits for all
upstream dependencies before executing.

Changes

• One-line fix in databricks_workflow.py
• Added unit test test_launch_task_waits_for_upstream_tasks
  validating that launch task correctly registers upstream task IDs

Testing

Unit test added in test_databricks_workflow.py covering the
external upstream dependency scenario from issue #51598.

Note on DAG Graph Screenshots

Previous PR #53559 was asked for before/after DAG graph screenshots.
Running into Windows/WSL encoding issues with Breeze setup.
Can provide screenshots if required during review — happy to
set up on a Linux environment if needed.

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