Queued prompt submitted twice (duplicate user message) in Copilot Chat agent sessions

### Summary

When prompts are **queued** in a Copilot Chat **agent session** (typed while the agent is still working on the current turn), a queued prompt is sometimes **submitted twice** — the identical user message is processed as **two consecutive turns**. The agent answers the request, and then immediately receives and processes the exact same prompt again.

This is a third distinct failure mode of queued-prompt handling in the agent host / Copilot CLI runtime, alongside the already-reported #317129 (queued message *disappears*) and #327649 (queued prompts *merged*). This one is **duplication**.

### Does this issue occur when all extensions are disabled?

N/A — the affected surface is Copilot Chat / the agent-session (Copilot CLI) runtime itself, which cannot be disabled to reproduce.

### Version

```
VS Code: 1.133.0
OS: WSL2 (Ubuntu 24.04.3 LTS) on Windows 10
Runtime: Copilot CLI runtime in VS Code (agent session / "agent host")
```

### Steps to Reproduce

1. Start an **agent session** (Copilot CLI runtime) and give it a task that takes a while (multiple tool calls).
2. **While the agent is still working**, type a follow-up prompt and press Enter so it is **queued** (repeat once or twice to build up a small queue).
3. Let the agent finish the current turn and pick up the queued prompt(s).

### Expected

Each queued prompt is delivered **exactly once**, as its own turn, in order.

### Actual

Intermittently, a queued prompt is delivered **twice**: the agent completes the request, then the **identical** user message arrives again as the next turn and is processed a second time. Observed roughly 6 times across a single long, queue-heavy session (i.e. intermittent, but common when queuing is used heavily).

Additional observed detail: in a couple of instances the **second (duplicate) copy carried different/broken attachment metadata** than the first — specifically a failed tagged-file reference of the form:

```
vscode-agent-host://wsl__ubuntu/.../Pasted%20Image.png (error: ENOENT ... 'vscode-agent-host://wsl__ubuntu/...')
```

Note the `wsl__ubuntu` (double-underscore) authority in that failed reference. This suggests the duplicate is a **separate re-delivery** of the queued prompt (the payload is re-resolved), not merely a UI echo — and that the re-delivery path mangles the remote authority (`wsl__ubuntu` instead of `wsl+ubuntu`). This may be related to the WSL remote authority handling seen in the agent-host "Open in VS Code" flow.

### Impact

- The agent redoes work it already completed (wasted tokens / tool calls, and potential double side effects if the prompt caused a mutation like a file edit, commit, or API call).
- Agents have to defensively detect "the previous message repeated verbatim" and no-op, which is unreliable and shouldn't be necessary.

### Related issues

- #317129 — Queued messages disappear for Copilot CLI (agent host)
- #327649 — Queued GitHub Copilot commands merged and use wrong model
- #330521, #326123 — queued-prompt feature requests

These all point at the queued-prompt dispatch path in the agent host; the duplication behavior here appears to be another edge case of the same mechanism.