fix: send unfiltered models over model/list (#11793)

### What
to unblock filtering models in VSCE, change `model/list` app-server
endpoint to send all models + visibility field `showInPicker` so
filtering can be done in VSCE if desired.

### Tests
Updated tests.

codex-rs/app-server/README.md

@@ -131,7 +131,7 @@ Example with notification opt-out:
 - `turn/interrupt` — request cancellation of an in-flight turn by `(thread_id, turn_id)`; success is an empty `{}` response and the turn finishes with `status: "interrupted"`.
 - `review/start` — kick off Codex’s automated reviewer for a thread; responds like `turn/start` and emits `item/started`/`item/completed` notifications with `enteredReviewMode` and `exitedReviewMode` items, plus a final assistant `agentMessage` containing the review.
 - `command/exec` — run a single command under the server sandbox without starting a thread/turn (handy for utilities and validation).
-- `model/list` — list available models (with reasoning effort options and optional `upgrade` model ids).
+- `model/list` — list available models (set `includeHidden: true` to include entries with `hidden: true`), with reasoning effort options and optional `upgrade` model ids.
 - `experimentalFeature/list` — list feature flags with stage metadata (`beta`, `underDevelopment`, `stable`, etc.), enabled/default-enabled state, and cursor pagination. For non-beta flags, `displayName`/`description`/`announcement` are `null`.
 - `collaborationMode/list` — list available collaboration mode presets (experimental, no pagination).
 - `skills/list` — list skills for one or more `cwd` values (optional `forceReload`).