Fix Classic Duo Chat UI is stuck

What does this MR do and why?

This MR fixes that users can't continue having a conversation in Classic Duo Chat because the loading never ends.

Since loading state has been removed from the store, this.isWaitingOnPrompt is updated at onMessageReceived callback in Classic Duo Chat component (which receives data from aiCompletionResponse GraphQL subscription hook) instead of using types.ADD_MESSAGE mutation handler. ref

Closes "... GitLab Duo is finding an answer" never go ... (#581270 - closed)

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Before	After
2025-11-19_14-07-17	2025-11-19_14-03-50

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