Implement AGV release logic to support append sub-tasks and holding vehicles

[grefus]

Background: Current implementation does not fully support workflows where AGVs are held at special stations and continued after sub-task append.

Problem Details:

1. For vehicles that are not released (held at special stations), logic does not reliably trigger next-task execution after appending a sub-task.
2. The relationship between holding status and sub-task append/continue is inconsistent and may result in idle AGVs or missed task transitions.
3. Existing logic in AppendSubTask and ReceiveTaskStatus needs revision to enable this workflow.

Expected Change:

• When AGVs are held (not released), ensure that an appended sub-task immediately continues the next task for the AGV without manual intervention.
• Consistently update the holding/release logic to trigger appropriate task execution upon sub-task append for vehicles at holding stations.
• Document any new state transitions or event handling required for these flows.

Impacts:

• Directly affects AppendSubTask and ReceiveTaskStatus methods.
• More robust AGV workflow at holding stations, reducing idle time.
• Potential downstream impacts for multi-vehicle workflows and sub-task state handling.

Reference: Review TaskService logic for handling AGV hold/release, sub-task append, and continue workflows.

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Action Items:

• Refactor AppendSubTask and ReceiveTaskStatus logic as described
• Ensure proper test cases for AGV holding points and sub-task continuation
• Update docs describing holding point and append sub-task flows