fix(client): enhance additional_headers input types

Author: anubhav756

packages/toolbox-adk/src/toolbox_adk/client.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Optional, Dict, Callable, Any, Union
+from typing import Optional, Dict, Callable, Any, Union, Awaitable
 import toolbox_core
 from google.auth import transport
 from google.auth import compute_engine
@@ -35,14 +35,15 @@ def __init__(
         self,
         server_url: str,
         credentials: Optional[CredentialConfig] = None,
-        additional_headers: Optional[Dict[str, str]] = None,
+
+        additional_headers: Optional[Dict[str, Union[str, Callable[[], str], Callable[[], Awaitable[str]]]]] = None,
         **kwargs
     ):
         """
         Args:
             server_url: The URL of the Toolbox server.
             credentials: The CredentialConfig object (from CredentialStrategy).
-            additional_headers: A dictionary of static headers to include in every request.
+            additional_headers: Dictionary of headers (static or dynamic callables).
             **kwargs: Additional arguments passed to toolbox_core.ToolboxClient.
         """
         self._server_url = server_url