Codex Agent Host fails in workspaces using only an extension-registered filesystem

### Description

Our extension registers a custom filesystem provider. When a VS Code workspace contains only resources from this extension-contributed filesystem, Codex Agent Host fails to start a turn with:

`Error: (CodexTurnError) Invalid request: AbsolutePathBuf deserialized without a base path`

The regular Codex extension handles the same workspace successfully.

**Steps to Reproduce**

- Create a new VS Code workspace.
- Add a file from an extension-registered filesystem provider.
- Ensure the workspace contains only resources from that filesystem.
- Enable: `"chat.editor.codex.preferAgentHost": true`
- Start a Codex Agent Host session and send Hello.
- Start a session using the regular Codex extension and send the same message.

**Actual Result**

Codex Agent Host fails with:
`Error: (CodexTurnError) Invalid request: AbsolutePathBuf deserialized without a base path`

The regular Codex extension starts and processes the request successfully.

**Expected Result**

Codex Agent Host should support workspaces backed by extension-registered filesystem providers, or gracefully handle non-native workspace URIs without passing an invalid or missing absolute path to Codex.

### Issue Details

Item|Value
---|---
Issue Category|Bug
Target|Visual Studio Code - Insiders
VS Code Version|Code - Insiders 1.137.0-insider (773da685a20213ce5755a80f523101669b145702, 2026-09-07T22:35:28Z)
OS Version|Windows_NT x64 10.0.26200

### Additional Issue Data

[issue-data.md](https://github.com/user-attachments/files/31952108/issue-data.md)

### Attachments