Add sample value generation context for Databinding and fix redirect

[RadCod3]

Purpose

This PR adds,

1. Context for AI-powered payload generation in event handler databinding, enabling the system to generate appropriate sample payloads for both HTTP and message broker services.
2. Redirect users to flow diagram after adding a new event handler (with or without databind).

Part of:

• Databinding support for handler functions on BI service designer view product-ballerina-integrator#1410

Depends on:

• Update service design meta info ballerina-platform/ballerina-language-server#422