Prevent in-flight task messages from recreating deleted tasks #2376

Summary

Investigate the shared task message lifecycle when a message write is already in progress during task disposal and deletion.

At PR #1403 head 1471730e6d22d1817e113c3df7d3a11a3ae273db, Task.addToClineMessages() awaits postStateToWebviewThrottled() before emitting the message event and calling saveClineMessages(). If disposal occurs during that await and another panel deletes the task, a late persistence operation may recreate the deleted task. The post-save diagnostics tail has cancellation guards before say() starts, but those guards cannot stop a say() already in progress.

Affected areas

• src/core/task/Task.ts: say(), ask(), addToClineMessages(), and task disposal.
• src/core/webview/ClineProvider.ts and task persistence/deletion paths.

Acceptance criteria

• Define and implement the disposal-versus-persistence contract at the appropriate shared boundary. Prevent a late in-flight message write from recreating a deleted task.
• Add a regression test that pauses the state post, disposes and deletes the task, then releases the post and checks that the task is not recreated.
• Verify normal say() and ask() event and persistence behavior, including unanswered ask ordering, remains correct.

Requested by @easonLiangWorldedtech.

Context: #1403
Review discussion: #1403 (comment)