feat: allow overriding default middleware configuration in create_deep_agent

[saiprasaad2002]

• Add override_default_middleware and custom_middleware parameters to create_deep_agent
• Preserve existing default middleware behavior when override is disabled
• Support fully custom middleware stacks (including custom models/params) while still wiring interrupt_on via HumanInTheLoopMiddleware

[eyurtsev]

If you're overriding the default middleware, this becomes a vanilla langchain agent, so use the vanilla agent for this use case.

[saiprasaad2002]

But what if I need to change the model for my middleware instead of keeping the same from my deep agent. Maybe I can prefer the summarization middleware to get gpt-5-mini with my deepagent having o3.

I was just exploring this repo for some days and kindly apologize me if i'm wrong here as this is my very first PR in my professional career. Still at the learning process..