Improve UI for natural language querying in visualization designer

What does this MR do and why?

Improves the UI for the natural language querying flow within the product analytics visualization designer.

• Adds "Experiment" badge
• Add descriptive paragraph
• Update button styling
• Change example placeholder prompt
• Caps textarea at 30% width inline with existing inputs

Note that error handling and form validation will follow in a future MR.

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
	(Loading)
	(Experiment badge open. Note: this badge + popup is an existing shared component)
	(large size display)

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. Enter ai prompts in the new UI

Related to #435786 (closed)