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/codex_app_server_protocol.v2.schemas.json

@@ -1972,6 +1972,30 @@
           "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": {
@@ -7689,6 +7713,32 @@
       "title": "ModelListResponse",
       "type": "object"
     },
+    "ModelProviderCapabilitiesReadParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "title": "ModelProviderCapabilitiesReadParams",
+      "type": "object"
+    },
+    "ModelProviderCapabilitiesReadResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "imageGeneration": {
+          "type": "boolean"
+        },
+        "namespaceTools": {
+          "type": "boolean"
+        },
+        "webSearch": {
+          "type": "boolean"
+        }
+      },
+      "required": [
+        "imageGeneration",
+        "namespaceTools",
+        "webSearch"
+      ],
+      "title": "ModelProviderCapabilitiesReadResponse",
+      "type": "object"
+    },
     "ModelRerouteReason": {
       "enum": [
         "highRiskCyberActivity"