feat: Add setdefault method to the ADK State object

This makes the ADK state more pythonic, with the State API being more like a python dict.

PiperOrigin-RevId: 796767559

src/google/adk/sessions/state.py

@@ -48,6 +48,14 @@ class State:
     """Whether the state dict contains the given key."""
     return key in self._value or key in self._delta
 
+  def setdefault(self, key: str, default: Any = None) -> Any:
+    """Gets the value of a key, or sets it to a default if the key doesn't exist."""
+    if key in self:
+      return self[key]
+    else:
+      self[key] = default
+      return default
+
   def has_delta(self) -> bool:
     """Whether the state has pending delta."""
     return bool(self._delta)