Fluent: sendbox transcript navigation

[OEvgeny]

Fixes #5153

Changelog Entry

• Added missing support for chat history scroll with keyboard when Fluent send box is focused, in PR 5191, by @OEvgeny

Description

This adds necessary handlers for transcript navigation into Fluent SendBox for feature parity with white label experience.

Design

Took this as an opportunity to reduce context usage. The subscriptions are now handled via use-paginate.

This adds the new useTranscriptNavigation private hook which currently returns only the keydown event handler, but can be further extended in the future. The handler is designed to be placed in the send box root and is intended to handle only usage patterns seen in the send box.

Specific Changes

• Reworked useScrollRelative and useRegisterScrollRelative
• Added transcript scroll with keyboard support into Fluent SendBox component
• Added tests

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)