chore: GenAI - Validating history in the ChatSession constructor

PiperOrigin-RevId: 590588154

Author: Ark-kun

vertexai/generative_models/_generative_models.py

@@ -655,6 +655,10 @@ def __init__(
         history: Optional[List["Content"]] = None,
         raise_on_blocked: bool = True,
     ):
+        if history:
+            if not all(isinstance(item, Content) for item in history):
+                raise ValueError("history must be a list of Content objects.")
+
         self._model = model
         self._history = history or []
         self._raise_on_blocked = raise_on_blocked