fix(forms): wire hover border on activator-based inputs

[kelsos]

Summary

• RuiAutoComplete, RuiMenuSelect, and RuiDateTimePicker share activatorStyles, which already defines an outlined + hovered border rule, but none of them tracked the hover state — so the hover-border-darken behavior present in RuiTextField/RuiTextArea was missing.
• Track isHovered via mouseenter/mouseleave on each activator and pass it into ui().
• Re-declare the hovered variant in activatorStyles for type inference, mirroring the existing filled re-declaration.

Test plan

• pnpm typecheck
• pnpm lint
• pnpm test:run for AutoComplete / MenuSelect / DateTimePicker
• Visual: hover the outlined variant of each component and confirm the border darkens (matching RuiTextField/RuiTextArea), and that the focus/error/success states still take precedence.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.83%. Comparing base (34a0ef1) to head (a91632b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
+ Coverage   84.79%   84.83%   +0.04%     
==========================================
  Files         145      145              
  Lines        5260     5269       +9     
  Branches     1571     1577       +6     
==========================================
+ Hits         4460     4470      +10     
+ Misses        800      799       -1     

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