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/schema/json/v2/ModelListResponse.json

@@ -31,6 +31,9 @@
         "displayName": {
           "type": "string"
         },
+        "hidden": {
+          "type": "boolean"
+        },
         "id": {
           "type": "string"
         },
@@ -71,6 +74,7 @@
         "defaultReasoningEffort",
         "description",
         "displayName",
+        "hidden",
         "id",
         "isDefault",
         "model",