Update types

Author: holtskinner

src/a2a/types.py

@@ -1151,7 +1151,9 @@ class MessageSendConfiguration(A2ABaseModel):
     Configuration for the send message request.
     """
 
-    accepted_output_modes: list[str] = Field(..., alias='acceptedOutputModes')
+    accepted_output_modes: list[str] | None = Field(
+        default=None, alias='acceptedOutputModes'
+    )
     """
     Accepted output modalities by the client.
     """