Secret filtering can block the CLI UI thread

### Describe the bug

Copilot CLI does secret scanning synchronously on the UI thread, which can freeze the TUI if taking too long. This can easily happen if the response objects are large because of the single threaded scanning. Also, scanning the response as structured objects with recursion is much slower than scanning by treating it as a single string.

In my situation, the freezing was worsened by the fact Copilot attempted to replay and submit past events when a session was resumed. Up to 500 events will be scanned for secrets at a time with scheduled batches, all happening on the UI thread and causing noticeable TUI freezing. What's more, if the remote submission fails, it will repeatedly scan the same 500 events for up to 5 times

### Affected version

1.0.64-3

### Steps to reproduce the behavior

Make some tool calls with large responses, and observe the TUI freezing

### Expected behavior

TUI shouldn't freeze regardless of the size of the responses or whether they are strings or structured data

### Additional context

_No response_

まず、CLI UI と、セッション再開のリプレイおよびバッチ送信フローで使用されている同期 secret-scanning パスを特定します。大きな構造化レスポンスと再開されたセッションでフリーズを再現し、その後、スキャンによって TUI がブロックされなくなったこと、また失敗した送信で同じイベントが繰り返し再スキャンされないことを確認します。

索引モデルが issue の本文から書いたものです。