docs(vertexai): document non-finite tool call coercion

Author: parkerhancock

libs/vertexai/langchain_google_vertexai/chat_models.py

@@ -622,6 +622,8 @@ def _append_to_content(
 
 
 def _json_safe_number(value: float) -> Union[str, float]:
+    # Gemini tool calls sometimes surface +/-inf or NaN; JSON forbids those, so
+    # coerce them to string markers before we hand them to json.dumps.
     if math.isnan(value):
         return "NaN"
     if math.isinf(value):