fix: prevent subtask restart on cancellation

[roomote]

Description

This PR fixes an issue where cancelling a subtask would immediately trigger it to restart instead of properly cancelling it.

Problem

When a user cancels a subtask, the parent task immediately restarts it because it doesn't know the subtask was cancelled by the user. This creates a frustrating experience where subtasks cannot be properly cancelled.

Solution

The fix introduces a mechanism to track when a subtask is cancelled by the user and communicate this to the parent task:

1. Added flag to the Task class to track cancelled subtasks
2. Modified method to accept a parameter
3. Updated in ClineProvider to:
   • Detect if the cancelled task is a subtask
   • Handle subtask cancellation differently from regular task cancellation
   • Notify the parent task that the subtask was cancelled (not just completed)
   • Avoid rehydrating the subtask (which would restart it)

Changes

• Modified to add cancellation tracking
• Updated to handle subtask cancellation properly
• Added comprehensive tests in

Testing

• ✅ Added 5 new test cases covering various cancellation scenarios
• ✅ All existing tests pass
• ✅ Manual testing confirms subtasks can now be cancelled without restarting

Related Issues

Fixes #8504

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Important

Fix subtask cancellation to prevent automatic restart and notify parent task, with tests added for verification.

• Behavior:
  • Cancelling a subtask now sets wasSubtaskCancelled in Task and notifies the parent task without restarting the subtask.
  • completeSubtask() in Task.ts updated to handle wasCancelled parameter.
  • finishSubTask() in ClineProvider.ts updated to pass wasCancelled flag.
• Testing:
  • Added ClineProvider.subtask-cancel.spec.ts with tests for subtask cancellation behavior.
  • Tests ensure no rehydration occurs for subtasks and proper handling of race conditions.
• Misc:
  • Minor logging adjustments in ClineProvider.ts.

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[roomote]

Self-reviewing my own code like a deterministic robot grading its dreams.

[roomote]

P1: Race condition and potential undefined access in the wait predicate. The predicate calls getCurrentTask() multiple times; the stack can change between calls, making a later call return undefined and then property access (e.g. .isStreaming) can throw. Snapshot the current task once per evaluation.

Example:

const predicate = () => {
const current = this.getCurrentTask();
return !current || current.isStreaming === false || current.didFinishAbortingStream || current.isWaitingForFirstChunk;
};

[roomote]

P3: New field wasSubtaskCancelled is written but not read anywhere. Consider either (a) using it to adjust parent control flow/logging/telemetry, or (b) deferring adding this state until it is consumed to avoid drift.

[roomote]

P3: Hardcoded user-facing string duplicated across files (also passed from ClineProvider.cancelTask). Consider centralizing or localizing via the existing i18n utilities (t("…")) to keep copy consistent and translatable.