feat(GlDuoChat): Adds empty state and prompt input placeholder props

What does this MR do?

This MR adds three new props to override previously hardcoded texts in the Duo Chat component.

1. emptyStateTitle
2. emptyStateDescription
3. chatPromptPlaceholder

Solves DuoChat: allow to override the empty state's ti... (#2453 - closed)

Also adds those props to the storybook.

Contributes to Chat interface for interacting with AI agents (gitlab#436471 - closed)

Screenshots or screen recordings

None, because this MR only allows customization of text that was previously hardcoded for the empty state component and the chat prompt input (placeholder).

See this integration screenshot in the current work on AI Agents where we're aiming to embed the Duo chat component to use it for a debug chat for the configured AI agent. Here all three texts have been overridden (some elements hidden/styled via CSS):

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

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Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.