fix(next-font): respect custom font-family declarations in local fonts (Turbopack)

[oskari]

What?

Fixes a bug in Turbopack where next/font/local with a custom font-family declaration would generate mismatched CSS:

• @font-face correctly used the custom declaration value (e.g., "My Custom Font")
• But .className incorrectly used the JS variable name (e.g., "myCustomFont")

This caused the font not to apply because the names didn't match.

Why?

When users specify a custom font-family in declarations:

const myCustomFont = localFont({
  src: './font.woff2',
  declarations: [{ prop: 'font-family', value: 'My Custom Font' }]
})

The generated CSS had a mismatch:

• @font-face { font-family: 'My Custom Font'; ... } ✅
• .className { font-family: 'myCustomFont', ... } ❌

Note: This bug only affects Turbopack. The Webpack implementation already handles this correctly via postcss-next-font.

How?

Updated the Turbopack implementation to check for custom font-family declarations when building:

1. util.rs: build_font_family_string now checks declarations for a custom font-family value before falling back to the variable name
2. font_fallback.rs: get_font_fallbacks now uses the custom font-family for fallback naming (e.g., "My Custom Font Fallback" instead of "myCustomFont Fallback")

Test Plan

• Added test fixture with custom font-family containing a space
• Added test verifying fontFamily matches the custom declaration
• Webpack: ✅ Already passes (not affected)
• Turbopack: ✅ Now passes with fix

Fixes #88894

[nextjs-bot]

Allow CI Workflow Run

• approve CI run for commit: 8a0b7e3

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

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_{Comparing oskari:fix/local-font-custom-family-declaration (8a0b7e3) with canary (acd5d5e)}

Footnotes

1. 3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩