Add backend client support for Codex user settings (#34559)

## What changed

- Add `Client::get_user_settings` for the active Codex API or ChatGPT backend route.
- Send authenticated workspace headers and bypass cached responses when fetching settings.
- Expose the effective `commit_attribution_enabled` policy, defaulting it to `false` when older responses omit the field.

## Testing

- Cover both backend route styles, cache-control behavior, response decoding, the compatibility default, and authenticated workspace headers.

GitOrigin-RevId: 11db485e89293f9fa9c56ac7997922deed8b9427

codex-rs/backend-client/Cargo.toml

@@ -28,3 +28,4 @@ codex-protocol = { workspace = true }
 [dev-dependencies]
 pretty_assertions = "1"
 tokio = { workspace = true, features = ["macros", "rt"] }
+wiremock = { workspace = true }