feat(chat-messages): add follow-up prompts

[robertwilde]

Closes #2066

Adds follow-up prompt support to SiChatInputComponent. A new followUpPrompts input (string[]) renders suggested next actions as pill buttons above the chat input; clicking a prompt inserts its text into the input field and emits a followUpPromptSelected output so the consuming application can react (e.g. clear the list). The si-chat-container example is updated to demonstrate the feature with three realistic prompts. Documentation is updated in both the chat input component page and the AI chat pattern page.

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Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request introduces follow-up prompts to the SiChatInputComponent, enabling users to select suggested actions that populate the chat input field. The implementation includes new input and output signals, template updates for rendering prompt buttons, and corresponding documentation and styling. Feedback from the review suggests using $index for tracking in the @for loop to avoid issues with duplicate strings, disabling prompt buttons while a message is being sent, and ensuring the textarea height is recalculated after a prompt is selected.

[robertwilde]

@panch1739 check this out :)

[panch1739]

@robertwilde Nice! One only note, the button should have a total height of 32px and the gap between them, 8px :)

Thank you so much!