[DRAFT] feat(prebuilt): add pre/post hooks for structured response in create_react_agent

[rickors560]

Description:
This PR adds pre_structured_response_hook and post_structured_response_hook parameters to the create_react_agent function, enabling preprocessing and post-processing around the generate_structured_response node for enhanced workflow extensibility.

What's added:

• pre_structured_response_hook: Optional hook executed before structured response generation
• post_structured_response_hook: Optional hook executed after structured response generation
• Updated workflow graph creation to properly route through the new hooks
• Comprehensive test coverage for all hook combinations with existing pre_model_hook and post_model_hook
• Updated interrupt handling to support the new hook nodes

Use Cases:

• Pre-hook: Message filtering, context preparation, input validation before structured output
• Post-hook: Response validation, transformation, logging, or error handling after structured generation

Implementation Details:

• Hooks are integrated as separate workflow nodes (not inline function calls)
• Maintains backward compatibility - all existing functionality works unchanged
• Proper edge routing ensures correct execution flow in all hook combinations
• Works with both tool-calling and non-tool-calling agent configurations

Issue: N/A (Enhancement request)

Dependencies: None

[vercel]

@rickors560 is attempting to deploy a commit to the LangChain Team on Vercel.

A member of the Team first needs to authorize it.

[rickors560]

Related Discussion: https://forum.langchain.com/t/add-pre-post-hooks-for-structured-response-generation-in-create-react-agent/1795

This is a draft PR to show the implementation approach discussed in the above thread.