Integrated browser remote proxy should support remote AHP hosts in Agents window

When an Agents window session is connected to a remote Agent Host Protocol (AHP) host, please allow integrated-browser traffic to be proxied through that session's remote host. This could be automatic for agent-opened tabs or an explicit per-tab **Browse through Agent Host** target.

## Why

The integrated browser can proxy through VS Code Remote workspace authorities, but a remote AHP host selected by an Agents window session is not treated as a proxy authority. As a result, an agent can run a service on its remote host but cannot open that service in the integrated browser without separately exposing or forwarding the port.

This also prevents secure loopback workflows where a browser needs to send an artifact directly to a one-use receiver on the selected agent host.

## Reproduction

Environment:

- Local client: VS Code 1.134.0 on macOS (integrated browser user agent: Electron 42.8.1 / Chrome 148)
- Remote AHP host: Windows, VS Code CLI/host 1.133.0
- Agents window session connected to that remote host
- Settings applied locally and the Agents window reloaded:

```json
"workbench.browser.enableRemoteProxy": true,
"workbench.browser.dataStorage": "workspace"
```

Steps:

1. On the remote Windows AHP host, start an HTTP server bound to `127.0.0.1:43127` that returns a unique nonce from `/probe`.
2. Verify from the remote host that `http://127.0.0.1:43127/probe` returns `200` and the nonce.
3. From the AHP-backed agent session, call the integrated-browser tool to open `http://127.0.0.1:43127/probe`.
4. Repeat with `http://localhost:43127/probe`.

Actual result:

- Both URLs fail with `net::ERR_CONNECTION_REFUSED`.
- The request never reaches the remote Windows server.
- The browser tab shows no remote indicator.
- The URL is resolved against the local macOS client, not the AHP host.

Expected result:

- The selected remote AHP host is available as a browser proxy target.
- Agent-opened tabs associated with that session can reach HTTP/HTTPS endpoints visible only to the AHP host.
- The browser shows the remote indicator and preserves per-session isolation.

## Related, but not duplicates

- #293229 implemented proxying through Remote Workspace connections via `workbench.browser.enableRemoteProxy`.
- #321294 tested that feature for Tunnels, SSH, Dev Containers, WSL, and Codespaces, including agent-opened tabs.
- #314789 tracks broad embedded-browser parity for remote Agent Hosts in the Agents window, but does not describe routing localhost/browser traffic through the selected AHP host.
- #324828 concerned a Remote Workspace BrowserView regression; disabling remote proxy fixed that issue. Here, remote proxy never activates for the AHP host at all.