Add missing using declarations for SKFont instances in SKFontTest

[Copilot]

Description of Change

27 test methods in SKFontTest.cs create new SKFont(...) without using, leaking native handles. The file already uses using var in 14 other tests. This adds the missing using keyword to all remaining instances.

// Before
var font = new SKFont(SKTypeface.Default);

// After
using var font = new SKFont(SKTypeface.Default);

Bugs Fixed

None.

API Changes

None.

Behavioral Changes

None.

Required skia PR

None.

PR Checklist

• Has tests (existing tests unchanged — only resource cleanup added)
• Rebased on top of main at time of PR
• Merged related skia PRs
• Changes adhere to coding standard
• Updated documentation