Fixes #4881: Preserve subtask completion state during task resumption

[roomote]

• Modified task resumption logic in Task.ts to detect completed subtasks
• Added check for finishTask tool completion when determining resume type
• Ensures completed subtasks show 'Start New Task' button instead of 'Resume Task'
• Prevents unnecessary re-execution of completed subtasks and token costs

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Important

Modifies task resumption logic in Task.ts to detect completed subtasks, ensuring correct UI display and preventing unnecessary re-execution.

• Behavior:
  • Modified task resumption logic in Task.ts to detect completed subtasks by checking for finishTask tool completion.
  • Ensures completed subtasks show 'Start New Task' button instead of 'Resume Task'.
  • Prevents unnecessary re-execution of completed subtasks and token costs.

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[daniel-lxs]

After reviewing this PR and discussing the issue, I think we're addressing a symptom rather than the root cause, also I tested it and it doesn't seem to work.

The real problem is that when a subtask is reopened after navigating away, the system loses context that it is a subtask, so the "Complete subtask and return to parent task" button does not appear. This happens whether the subtask is in progress or already completed.

The current PR checks for finishTask tool calls to detect completed subtasks, but that only helps in those cases. If the subtask is still in progress, the UI falls back to showing "Cancel" and "Resume Task" instead of the subtask-specific controls.

The core issue is that the subtask UI state is not preserved when reopening subtasks. The fix should ensure this context is maintained for all subtasks when resumed, not just completed ones.

I will add a more detailed analysis and proposed solution to the original issue (#4881).