fix: handle JSON Schema in additionalProperties for MCP tools (#4454)

Fixes #4176

Some common tools provide a schema (even if just an empty object schema)
as the value for `additionalProperties`. The parsing as it currently
stands fails when it encounters this. This PR updates the schema to
accept a schema object in addition to a boolean value, per the JSON
Schema spec.

codex-rs/core/src/exec_command/responses_api.rs

@@ -49,7 +49,7 @@ pub fn create_exec_command_tool_for_responses_api() -> ResponsesApiTool {
         parameters: JsonSchema::Object {
             properties,
             required: Some(vec!["cmd".to_string()]),
-            additional_properties: Some(false),
+            additional_properties: Some(false.into()),
         },
     }
 }
@@ -92,7 +92,7 @@ Can write control characters (\u0003 for Ctrl-C), or an empty string to just pol
         parameters: JsonSchema::Object {
             properties,
             required: Some(vec!["session_id".to_string(), "chars".to_string()]),
-            additional_properties: Some(false),
+            additional_properties: Some(false.into()),
         },
     }
 }