Support anyOf and enum in JsonSchema (#16875)

This brings us into better alignment with the JSON schema subset that is
supported in
<https://developers.openai.com/api/docs/guides/structured-outputs#supported-schemas>,
and also allows us to render richer function signatures in code mode
(e.g., anyOf{null, OtherObjectType})

codex-rs/app-server/src/codex_message_processor.rs

@@ -9138,6 +9138,24 @@ mod tests {
         validate_dynamic_tools(&tools).expect("valid schema");
     }
 
+    #[test]
+    fn validate_dynamic_tools_accepts_nullable_field_schema() {
+        let tools = vec![ApiDynamicToolSpec {
+            name: "my_tool".to_string(),
+            description: "test".to_string(),
+            input_schema: json!({
+                "type": "object",
+                "properties": {
+                    "query": {"type": ["string", "null"]}
+                },
+                "required": ["query"],
+                "additionalProperties": false
+            }),
+            defer_loading: false,
+        }];
+        validate_dynamic_tools(&tools).expect("valid schema");
+    }
+
     #[test]
     fn config_load_error_marks_cloud_requirements_failures_for_relogin() {
         let err = std::io::Error::other(CloudRequirementsLoadError::new(