feat(DuoChat): UI Polish on context

What does this MR do?

Small styling and layout changes for Duo chat context files

Changes:

1. Collapse references by default on user and assistant messages
2. Move user message references below message
3. Style user message references to better match user message style (and to improve accessible contrast)
4. Move target of popover to token, not text within token to adjust positioning of tooltip tail
5. Adjust style and layout of tooltip content (styling inspired by/meant to match gitlab issue/MR/Epic mention popover)
6. Use alert style for alert notes in file popover

Screenshots or screen recordings

Description	Before	After
default state
all expanded state
user-side file
user-side issue
agent-side included reference
MR (showing warning note)

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.