feat: expose provider capability bounds to app server clients (#20049)

follow up of #19442. The app server now exposes provider-derived bounds
through a new v2 `modelProvider/read` method. The response reports the
configured provider map key as `modelProvider` and returns the effective
capability booleans so clients can align their UI with the same
provider-owned limits used by core.

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -1896,6 +1896,9 @@
       },
       "type": "object"
     },
+    "ModelProviderCapabilitiesReadParams": {
+      "type": "object"
+    },
     "NetworkAccess": {
       "enum": [
         "restricted",
@@ -5478,6 +5481,30 @@
       "title": "Model/listRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "modelProvider/capabilities/read"
+          ],
+          "title": "ModelProvider/capabilities/readRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/ModelProviderCapabilitiesReadParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "ModelProvider/capabilities/readRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {