Allow reordering queued messages before they run

### Feature summary

Allow queued follow-up messages to be moved up or down while they are still pending.

### What problem are you trying to solve?

I often queue several follow-up messages while Copilot is working. I have encountered situations where a later message becomes a prerequisite or should be handled before messages that were queued earlier.

For example:

1. Start a task and let Copilot continue working.
2. Queue messages A, B, and C.
3. Realize that C must be handled before A.
4. Open the queue and try to change the order.

Actual: I can view the queued messages, but I cannot change their order.

Expected: I can move C before A without interrupting the active work or recreating the messages.

### Proposed solution

Allow messages that are still queued to be reordered.

This could support drag-and-drop as well as Move up and Move down actions for keyboard accessibility. Reordering should not interrupt the active turn or change the message content or attachments.

Once a message has started, it should no longer be reorderable.

### Workflow impact

This would let me arrange queued work into the correct priority or dependency order as new information appears, without steering the active task or waiting for messages to run in the wrong order.

### Installation context

GitHub Copilot app 1.0.23 on Windows x64, OS build 26200.8655, in the /chat view.

### Additional context

Related: #534 covers editing and canceling queued messages, #1373 covers removing or sending them immediately, and #1549 covers canceling a pending message. This request is specifically about changing the order of messages that are still queued.

GitHub Copilot Chat in VS Code already supports reordering pending messages:
https://code.visualstudio.com/docs/chat/chat-overview#_reorder-pending-messages

从 /chat 视图开始，追踪排队的后续消息是如何显示和分派的；该 issue 未指出任何 repository 文件或测试。对比文档中 VS Code 对待处理消息重新排序的行为。完成标准是：排队的消息可以向上或向下移动，而不会影响活动中的工作、消息内容或附件；已开始的消息则不能移动。

由索引模型根据 Issue 内容生成。