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/codex_app_server_protocol.schemas.json

@@ -12611,6 +12611,9 @@
           "displayName": {
             "type": "string"
           },
+          "hidden": {
+            "type": "boolean"
+          },
           "id": {
             "type": "string"
           },
@@ -12651,6 +12654,7 @@
           "defaultReasoningEffort",
           "description",
           "displayName",
+          "hidden",
           "id",
           "isDefault",
           "model",
@@ -12668,6 +12672,13 @@
               "null"
             ]
           },
+          "includeHidden": {
+            "description": "When true, include models that are hidden from the default picker list.",
+            "type": [
+              "boolean",
+              "null"
+            ]
+          },
           "limit": {
             "description": "Optional page size; defaults to a reasonable server-side value.",
             "format": "uint32",