docs: Update graph-api.md for "Extended example: specifying LLM at runtime" (#5938)

docs (graphapi) : Handle Missing Context in LLM Invocation - Invoking
the LLM without explicitly passing a context parameter resulted in the
following error:
`AttributeError: 'NoneType' object has no attribute 'model_provider'`
This occurred because the context was None, and the system attempted to
access model_provider. This PR ensures that when context is not
provided, an empty context is passed explicitly. This allows the system
to correctly fall back to the default value defined in the
ContextSchema.model_provider attribute.

docs/docs/how-tos/graph-api.md

@@ -1019,7 +1019,7 @@ console.log(await graph.invoke({}, { configurable: { myRuntimeValue: "b" } }));
     # Usage
     input_message = {"role": "user", "content": "hi"}
     # With no configuration, uses default (Anthropic)
-    response_1 = graph.invoke({"messages": [input_message]})["messages"][-1]
+    response_1 = graph.invoke({"messages": [input_message]}, context=ContextSchema())["messages"][-1]
     # Or, can set OpenAI
     response_2 = graph.invoke({"messages": [input_message]}, context={"model_provider": "openai"})["messages"][-1]