chore: Add icon size and stroke width props

[at-susie]

Description

This PR adds:

1. New sizes and strokeWidths props on IconProvider:

Extend the existing IconProvider component with two new props — sizes and strokeWidths — that allow overriding icon dimensions and stroke widths at any level of the component tree.
In a nutshell, this proposal enables icon size and stroke-width customization by overriding the inline size and stroke-width of both the icon SVG and its wrapper element via the existing IconProvider.

Because the icon-provider already establishes a predictable scoping mechanism, icon sizes can be flexibly adjusted at different levels of the UI while maintaining consistent behavior.
This approach:

• Leverages the existing provider/context pattern used for icon customization
• Supports scoped overrides (global → local)
• Provides independent control over both size and stroke-width per variant

2. A new x-small icon size variant (12px) for the icon component:

While it’s not currently used by any existing components, it supports future use cases we’ve already identified, such as icons inside badge components and icons used in button groups within prompt inputs.

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

❌ Patch coverage is 88.23529% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.41%. Comparing base (7c2631d) to head (fe62fc1).

Files with missing lines	Patch %	Lines
src/icon-provider/internal.tsx	81.57%	7 Missing ⚠️
src/icon/internal.tsx	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4514      +/-   ##
==========================================
- Coverage   97.42%   97.41%   -0.02%     
==========================================
  Files         939      939              
  Lines       29645    29708      +63     
  Branches    10772    10800      +28     
==========================================
+ Hits        28883    28939      +56     
- Misses        715      722       +7     
  Partials       47       47              

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