feat: runnable generator

[zabealbe]

Add Support for RunnableGenerator in LangChain.js

Summary

This PR addresses an issue where streaming stops working when using .withStructuredOutput() followed by .pipe() to transform the output structure in LangChain.js. The stream should continue emitting results as expected, but the current implementation causes it to only return the last chunk.

Background & Motivation

Issue: #4936

• When using a LangChain model with .withStructuredOutput() and then applying .pipe() to transform the output structure, streaming no longer works.
• The issue arises because .pipe() internally creates a RunnableLambda, which buffers all results before invoking the function, preventing the incremental emission of outputs required for streaming.
• Example of broken behavior:

const model = new ChatOpenAI().withStructuredOutput<MyOutputType>();

const mappedChain = model.pipe((output: MyOutputType) => ({
  newField: output.oldField,
}));

for await (const chunk of mappedChain.stream({ input: "Hello" })) {
  console.log(chunk); // Logs only the last chunk
}

Key Changes

• Implemented RunnableGenerator to handle streaming transformations without accumulating input.

const model = new ChatOpenAI().withStructuredOutput<MyOutputType>();

const mappedChain = model.pipe(new RunnableGenerator({
  func: (output: MyOutputType) => ({
    newField: output.oldField,
  })
});

for await (const chunk of mappedChain.stream({ input: "Hello" })) {
  console.log(chunk); // Works
}

Expected Impact

• Streaming will now work when transforming structured outputs using .pipe().
• Simplifies handling streaming transformations, reducing complexity for users.
• Improves real-time processing for cases where output structures need to be modified dynamically.
• LangChain for Python already includes RunnableGenerator, but this feature was missing in LangChain.js.

Fixes

Fixes #4936

🚧 Draft: Needs Testing 🚧

This is a draft PR and still needs testing to ensure everything works as expected in all edge cases. Please help by running some tests and sharing feedback!

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Name	Status	Preview	Comments	Updated (UTC)
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