fix code cell for streaming

[ccurme (ccurme)]

Currently we split the streaming loop and combination into separate code blocks.

There are a few problems with the code block for combining chunks:

1. It doesn't work (we sum an AIMessage, which results in a ChatPromptTemplate):

from langchain_core.messages import AIMessage, AIMessageChunk

chunks = [AIMessageChunk("foo"), AIMessageChunk(" bar")]

final_message = sum(chunks, AIMessage(""))  # ChatPromptTemplate

2. More nitpicky, but I think aggregating chunks is important and also concise enough that we can display it in the first cell with just two additional lines.

Here we demonstrate aggregation in the first cell (and also remove an import).

[Copilot]

Pull Request Overview

This PR fixes a broken code example for streaming message aggregation in the LangChain documentation. The main issue was that the existing code for combining chunks used an incorrect pattern that resulted in a ChatPromptTemplate instead of an AIMessage.

• Consolidates streaming and chunk aggregation into a single, working code block
• Removes the broken sum(chunks, AIMessage("")) pattern that produces incorrect results
• Demonstrates proper streaming aggregation inline with collection

[github-actions]

Preview ID generated: preview-ccfixs-1758202838-928639b

[github-actions]

Preview ID generated: preview-ccfixs-1758220102-e6cc573