chore: Change Pydantic model config to ignore extra fields in Event

Update the `model_config` for `Event` to set `extra='ignore'`. This allows the `Event` model to be initialized with fields not explicitly defined in the model, preventing errors when deserializing data that may contain additional, unneeded fields. This allows the session service to accept 2.0 events.

Co-authored-by: Shangjie Chen <deanchen@google.com>
PiperOrigin-RevId: 889327445

Author: DeanChensj

src/google/adk/events/event.py

@@ -36,7 +36,7 @@ class Event(LlmResponse):
   """
 
   model_config = ConfigDict(
-      extra='forbid',
+      extra='ignore',
       ser_json_bytes='base64',
       val_json_bytes='base64',
       alias_generator=alias_generators.to_camel,