Warn when generating over the top of existing visualization changes

What does this MR do and why?

When generating a query using Duo natural language querying, it replaces the existing selections (e.g. measure, dimensions) with whatever was generated from the model. This MR introduces a confirmation for the user before running the prompt to ensure they do not unexpectedly lose work.

This MR builds on previous error handling MR: Add error handling for visualization designer n... (!144146 - merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
	(unchanged when viz designer is in pristine state)
(generates and replaces without confirming)	(confirms before generating)

How to set up and validate locally

Apply setup steps for backend MR this one depends on:

• Enable AI features locally: https://docs.gitlab.com/ee/development/ai_features/#test-ai-features-locally
• Make sure you've enabled Vertex access and configured it with your GDK. This action does not use Anthropic's API, only Vertex.
• Enable the :generate_cube_query flag locally. NEW

Then:

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Select some measures/dimensions manually using the UI
5. Type in a prompt and click "Generate with Duo"
6. Verify confirmation prompt works as expected

Related to #441694 (closed)