fix: Make truncated filename tooltip keyboard accessible (file-token-group)

[Who-is-PS]

Description

Adds keyboard accessibility to the file token tooltip. The filename container is now focusable (tabIndex={0}) with onFocus/onBlur handlers, allowing keyboard users to view the full filename when text is truncated.

Related links, issue #, if available:
AWSUI-61600

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.14%. Comparing base (1a5715e) to head (9afbafd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4157   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files         873      873           
  Lines       25642    25644    +2     
  Branches     9284     9284           
=======================================
+ Hits        24910    24912    +2     
  Misses        726      726           
  Partials        6        6           

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