fix(DuoChat): ensure chat scrolls to bottom with new message in all contexts

What does this MR do?

Ensures Duo Chat scrolls to bottom with new messages, instead of when the loading state changes.

Previously, scrollToBottom was triggered based on loading state change. This caused problems within VSCode, where scrolling was not triggered when initial chat history was restored (because VSCode renders the component, then asynchronously restores chat history).

We now listen for changes to lastMessage, and trigger scrolling on that.

To prevent users scroll position being hijacked if they have explicitly scrolled up to read an earlier message, we only maintain scrolling to bottom if the chat is already scrolled to the bottom.

Screenshots or screen recordings

in VSCode	in Storybook

Note how scrolling isn't impacted when scrolled up to read an earlier message.

Integration merge requests

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

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• 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

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

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.