Prevent instantiating more than one ChatAssistant per UI #59

Feature Request

It is not the intended design of the add-on to have two ChatAssistant instances loaded at the same time on the same page/UI. This should be enforced at the component level to prevent misuse and avoid subtle bugs (e.g., shared global state in client-side listeners).

Problem

Currently, nothing prevents a developer from adding multiple ChatAssistant instances to the same Vaadin UI. This can lead to unexpected behavior, including conflicts in client-side JavaScript markers (e.g., document[flag] collisions) and overlapping event listeners.

Proposed Solution

Add a guard in ChatAssistant that checks whether an instance is already registered for the current Vaadin UI and either:

• Throws an IllegalStateException if a second instance is attached, or
• Logs a warning and prevents initialization of the duplicate.

This check could be implemented in the onAttach lifecycle method by storing a UI-scoped reference.

References

• Related PR: Drop React FAB dependency by replacing ReactAdapter with vanilla component  #58
• Review comment: Drop React FAB dependency by replacing ReactAdapter with vanilla component  #58 (comment)
• Requested by: @mlopezFC