Remote session artifact fails to open because its Windows path is resolved locally

- A Markdown artifact created by an agent on a remote Windows host cannot be opened from the macOS Sessions UI.

### Steps to reproduce
1. From macOS, open a completed Copilot agent session running on a remote Windows host through a tunnel.
2. Click the Markdown artifact chip in the final response.
3. Observe the editor error.

- **Expected:** The artifact opens through the remote agent host.
- **Actual:** VS Code tries to read the remote `c:/Users/...` path locally and reports that the file does not exist.
- **Environment:** VS Code Insiders 1.137.0-insider on macOS 26.6.2; remote Windows host; Copilot Chat 0.65.2026090401; Agent mode; GPT-6 Astra.
- **Extensions disabled:** Not checked.

Details for investigation

- The tunnel response at `05:43:45.969Z` successfully resolved the artifact as a file on the remote host with size `29621` bytes:

```text
file:///c%3A/Users//.copilot/session-state//files/wsl-byok-support-plan.md
```

- At `22:44:11` local time, the renderer then attempted to open the Windows path directly and failed:

```text
Unable to read file 'c:/Users//.copilot/session-state//files/wsl-byok-support-plan.md'
(Error: Unable to resolve nonexistent file ...)
```

- The same renderer error repeated at `22:44:27` and `22:44:42`.