AI: Improve AccessibilityAgent preamble and add context tools

This CL improves the AccessibilityAgent by updating its preamble to
be more structured and proactive. It also adds new tools that allow
the agent to fetch live page data using Lighthouse paths.

- Updated the preamble to enforce a "problem, root cause, next steps"
  structure and prioritize critical issues.
- Added `getStyles` to fetch computed styles for elements identified
  in the Lighthouse report. This function is similar to the one in the
  StylingAgent, but it uses a different look-up system as LH only has
  the path not the backend node IDs.
- Added `getElementAccessibilityDetails` to fetch live accessibility
  properties (role, name, ARIA attributes) using the AccessibilityModel.
- Implemented a private helper to resolve Lighthouse paths to DOM nodes.
- Added unit tests for the new functionality.

The preamble will need syncing to the server, and also I'm still testing
how often the agent calls these functions, but I wanted to land this as
as starting point.

Bug: 489620101
Change-Id: I978de38a189f848991a5999e5e69b7ba158cef46
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/7679406
Reviewed-by: Alina Varkki <alinavarkki@chromium.org>
Auto-Submit: Jack Franklin <jacktfranklin@chromium.org>
Commit-Queue: Alina Varkki <alinavarkki@chromium.org>