fix(app): keep workspace message text readable on accent themes - #51528

Issue for this PR

Closes #50986

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Workspace user messages use the theme accent background, but paired it with a foreground meant for a different surface. One Dark Pro dark mode therefore rendered the message at about 1.1:1 contrast. I added an accent foreground token chosen against the resolved accent background and used it for the workspace message and mention prefix. This keeps the accent styling while choosing readable text for bright as well as dark accents. The selector and contrast diagnosis were documented in the issue discussion.

How did you verify your code works?

• Current head merged v2 at de2cacd; upstream pending-message selectors and spacing remain intact.
• bun test src/theme/v2/resolve.test.ts in packages/ui: 84 passed, 99 assertions, covering built-in color schemes, custom fallback and an overridden accent surface. git diff --check passed.
• bun run check through the unmodified pre-push hook passed: full lint and SDK documentation checks, then 36/36 typecheck tasks. Astro diagnostics passed despite sandbox log-directory warnings.
• Rendered an actual workspace message in the local app using One Dark Pro dark mode. Text is readable on the blue accent surface. The provider rejected the optional response request; this screenshot verifies message rendering, not a successful model response.

Screenshots / recordings

Before: Issue screenshot.

After, current code with One Dark Pro dark mode:

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR