core[patch]: revert change to stream type hint

[ccurme]

#31286 included an update to the return type for BaseChatModel.(a)stream, from Iterator[BaseMessageChunk] to Iterator[BaseMessage].

This change is correct, because when streaming is disabled, the stream methods return an iterator of BaseMessage, and the inheritance is such that an BaseMessage is not a BaseMessageChunk (but the reverse is true).

However, LangChain includes a pattern throughout its docs of summing BaseMessageChunks to accumulate a chat model stream. This pattern is implemented in tests for most integration packages and appears in application code. So #31286 introduces mypy errors throughout the ecosystem (or maybe more accurately, it reveals that this pattern does not account for use of the .stream method when streaming is disabled).

Here we revert just the change to the stream return type to unblock things. A fix for this should address docs + integration packages (or if we elect to just force people to update code, be explicit about that).

[vercel]

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1 Skipped Deployment

Name	Status	Preview	Comments	Updated (UTC)
langchain	⬜️ Ignored (Inspect)			Jun 5, 2025 3:13pm

[codspeed-hq]

CodSpeed Walltime Performance Report

Merging #31501 will not alter performance

_{Comparing cc/stream_types (a5c7a4d) with master (b149cce)}

⚠️ Unknown Walltime execution environment detected

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For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 13 untouched benchmarks

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #31501 will not alter performance

_{Comparing cc/stream_types (a5c7a4d) with master (b149cce)}

Summary

✅ 13 untouched benchmarks