core/app-server: preflight image modality mismatch with typed error

codex-rs/app-server-protocol/schema/json/v2/ThreadStartedNotification.json

@@ -65,6 +65,31 @@
           "title": "ModelCapCodexErrorInfo",
           "type": "object"
         },
+        {
+          "additionalProperties": false,
+          "properties": {
+            "unsupportedInputModality": {
+              "properties": {
+                "modality": {
+                  "$ref": "#/definitions/InputModality"
+                },
+                "model": {
+                  "type": "string"
+                }
+              },
+              "required": [
+                "modality",
+                "model"
+              ],
+              "type": "object"
+            }
+          },
+          "required": [
+            "unsupportedInputModality"
+          ],
+          "title": "UnsupportedInputModalityCodexErrorInfo",
+          "type": "object"
+        },
         {
           "additionalProperties": false,
           "properties": {
@@ -368,6 +393,25 @@
       },
       "type": "object"
     },
+    "InputModality": {
+      "description": "Canonical user-input modality tags advertised by a model.",
+      "oneOf": [
+        {
+          "description": "Plain text turns and tool payloads.",
+          "enum": [
+            "text"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "Image attachments included in user turns.",
+          "enum": [
+            "image"
+          ],
+          "type": "string"
+        }
+      ]
+    },
     "McpToolCallError": {
       "properties": {
         "message": {