feat(langchain): Support conditional construction of agent middleware with AgentMiddlewareBuilder

[carefreelife98]

Summary

Agent creation often requires toggling multiple middleware components (todo list, human-in-the-loop, prompt caching, etc.) based on runtime conditions. Today every caller must import each factory and manually push them into arrays, which leads to duplicated logic and error-prone configuration handling.

Proposal

1. Expose a centralized factory enum (BuiltInAgentMiddleware) plus createBuiltInAgentMiddleware so callers can request middleware by name.
2. Provide a lightweight AgentMiddlewareBuilder that accepts descriptors { name, config, when } and returns the assembled array, mirroring the builder used in Mate-X.
3. Update docs/examples to illustrate building agent middleware stacks with conditional guards.
4. Add unit tests ensuring the builder respects when flags and that factory entries require the correct config shape.

Motivation

• Improves ergonomics for teams composing agents with many optional behaviors.
• Avoids repeated imports and misconfiguration (e.g. missing retry configs).
• Gives us a single extension point when introducing new built-in middleware.

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