[Bug]"Add File to Chat" silently does nothing in the Agents Window

Does this issue occur when all extensions are disabled?: Yes

- VS Code Version: 1.131.0
- OS Version: macOS

### Steps to Reproduce

1. Open the **Agents Window** (`Workbench: Open Agents Window` / `workbench.action.openAgentsWindow`) and open an existing agent session so the chat view is visible.
2. Trigger **"Chat: Add File to Chat"** (`workbench.action.chat.attachFile`) — e.g. from the Command Palette, an editor context menu, or the Explorer context menu.

**Actual result:** Nothing happens. No attachment appears in the chat input of the active session, and no notification or error is shown — the action silently no-ops.

**Expected result:** The selected file is attached to the active session's chat input in the Agents Window (same behavior as the chat view in a regular editor window).