fix(a11y): improve focus visibility on suggested terms

[CookieDarb]

What?

Closes #570

This PR improves the focus visibility of the suggested term and dismiss buttons inside the Suggested Terms taxonomy pills for keyboard users.

Why?

Previously, keyboard users could not discern which button within a suggested taxonomy pill had focus. This improves accessibility and focus clarity.

How?

Added explicit focus styles to the suggested-term action buttons so the focused state is clearly visible, using existing WordPress admin theme variables for consistency.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Model(s): Gemini 3.1 Pro, Gemini 3 Flash
Used for: Suggesting and implementing the fix. Final implementation was reviewed and edited by me.

Testing Instructions

1. Open a post with content (about 150+ words).
2. Click “Suggest Categories” to show suggested terms.
3. Use the keyboard (Tab/Shift+Tab) to focus the buttons.

Screenshots or screencast

Add button is focused:

Dismiss button is focused:

Changelog Entry

Fixed - Improve keyboard focus visibility for suggested term actions in content classification.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.41%. Comparing base (1704050) to head (a17075c).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #580   +/-   ##
==========================================
  Coverage      71.41%   71.41%           
  Complexity      1158     1158           
==========================================
  Files             67       67           
  Lines           5586     5586           
==========================================
  Hits            3989     3989           
  Misses          1597     1597           

Flag	Coverage Δ
unit	71.41% <ø> (ø)

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[github-actions]

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Co-authored-by: CookieDarb <sakshamsharma5@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[jeffpaul]

Tested and works as expected/described, thanks!