fix(backend): fixes DAG status update to reflect completion of all tasks

[VaniHaripriya]

Description of your changes:

This PR addresses the following issue:
Subdags are being updated as soon as the first task within the subdag completes, which is incorrect behavior. The subdag status should only be updated once all tasks have completed.

This fix ensures that the subdag status is updated only after all tasks have been executed.

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[VaniHaripriya]

DAG_Status_Update_Fix.mp4

Attaching a sample workflow after the fix.

[zazulam]

@VaniHaripriya can you confirm that this change works for the oneof subdag example?

[VaniHaripriya]

/retest