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

Start in the /chat view and trace how queued follow-up messages are displayed and dispatched; the issue names no repository files or tests. Compare the documented VS Code pending-message reordering behavior. Done means queued messages can move up or down without affecting active work, message content, or attachments, while started messages cannot move.

Written by the indexing model from the issue text.