fix: undefined field type (#1031)

# Motivation

<!-- Why is this change necessary? -->

# Content

<!-- Please include a summary of the change -->

# Testing

<!-- How was the change tested? -->

# Please check the following before marking your PR as ready for review

- [ ] I have added tests for my changes
- [ ] I have updated the documentation or added new documentation as
needed

---------

Co-authored-by: rushilpatel0 <[email protected]>

Author: rushilpatel0

src/codegen/agents/client/openapi_client/api_response.py

@@ -2,21 +2,18 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Generic, TypeVar
+from typing import Generic, TypeVar
 
 from pydantic import BaseModel, Field, StrictBytes, StrictInt
 
-if TYPE_CHECKING:
-    from collections.abc import Mapping
-
 T = TypeVar("T")
 
 
 class ApiResponse(BaseModel, Generic[T]):
     """API response object"""
 
     status_code: StrictInt = Field(description="HTTP status code")
-    headers: Mapping[str, str] | None = Field(None, description="HTTP headers")
+    headers: dict[str, str] | None = Field(None, description="HTTP headers")
     data: T = Field(description="Deserialized data given the data type")
     raw_data: StrictBytes = Field(description="Raw data (HTTP response body)")