Agentic Chat: Allow users to resume/continue previous chat sessions instead of forcing new chat creation

What does this MR do?

Previously, when a user canceled a chat session, the workflowID would be reset along with other state. This meant that if the user started a new message after canceling, a completely new workflow would be created.

This MR modifies the cleanupState() method to optionally preserve the workflowID when canceling a chat, allowing the conversation to continue in the same workflow context.

solves: #555580 (closed)

before	after
2025-07-15_15-22-39	2025-07-15_15-24-26

How to reproduce:

• Setup Agentic Chat in your GDK !197267 (merged)
• Ask Agentic Chat a question, cancel the request, ask it if it remembers.