Add `.invoke()` Method to Graph API (LangGraph-like Execution)

[divinoborges]

Summary

Currently, Graph only exposes a .launch() method, which is great for interactive and UI-based workflows (e.g. Gradio apps). However, this design limits the use of Graph in programmatic, server-side, and API-driven contexts.

This issue proposes introducing a .invoke() method, similar to how LangGraph works, enabling graphs to be executed as pure functions and easily exposed as APIs.

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Motivation

Many real-world use cases require graphs to be:

• Executed headlessly (without UI)
• Integrated into REST / FastAPI / serverless APIs
• Used in pipelines, backends, or microservices
• Composable and callable like regular Python functions