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/src/protocol/common.rs

@@ -642,6 +642,11 @@ client_request_definitions! {
         serialization: None,
         response: v2::ModelListResponse,
     },
+    ModelProviderCapabilitiesRead => "modelProvider/capabilities/read" {
+        params: v2::ModelProviderCapabilitiesReadParams,
+        serialization: None,
+        response: v2::ModelProviderCapabilitiesReadResponse,
+    },
     ExperimentalFeatureList => "experimentalFeature/list" {
         params: v2::ExperimentalFeatureListParams,
         serialization: global("config"),
@@ -2243,6 +2248,23 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn serialize_model_provider_capabilities_read() -> Result<()> {
+        let request = ClientRequest::ModelProviderCapabilitiesRead {
+            request_id: RequestId::Integer(7),
+            params: v2::ModelProviderCapabilitiesReadParams {},
+        };
+        assert_eq!(
+            json!({
+                "method": "modelProvider/capabilities/read",
+                "id": 7,
+                "params": {}
+            }),
+            serde_json::to_value(&request)?,
+        );
+        Ok(())
+    }
+
     #[test]
     fn serialize_list_collaboration_modes() -> Result<()> {
         let request = ClientRequest::CollaborationModeList {

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -2355,6 +2355,20 @@ pub struct GetAccountResponse {
     pub requires_openai_auth: bool,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, Default, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ModelProviderCapabilitiesReadParams {}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ModelProviderCapabilitiesReadResponse {
+    pub namespace_tools: bool,
+    pub image_generation: bool,
+    pub web_search: bool,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]