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-protocol/src/protocol/common.rs

@@ -1235,7 +1235,8 @@ mod tests {
                 "id": 6,
                 "params": {
                     "limit": null,
-                    "cursor": null
+                    "cursor": null,
+                    "includeHidden": null
                 }
             }),
             serde_json::to_value(&request)?,