Fix: Correct font slug before calculating $to_add

[enodekciw]

What?

Incorrect font-family slug is being used while registering theme webfonts.

Why?

Solves #46289

How?

Passing $family to WP_Webfonts::get_font_slug( $family ) returns sanitized font-family. This is OK for webfonts, but not OK when used on settings.typography.fontFamilies. Also, there's already $family['slug'] available to check against.

[github-actions]

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[simison]

Added few reviewers to move this forward; feel free to ping others or un-assign yourself if it makes sense.

[t-hamano]

Sorry, I am not familiar with the web font codebase and cannot determine if this change is appropriate.

@aristath
I would be happy if you will look into this PR 🙏

[matiasbenedetto]

Thanks a lot, @enodekciw, for working on this!

I think this approach you propose breaks in certain cases, specifically when the font family doesn't have a slug defined. It doesn't solve either this related issue around duplicated fonts when using fallback values in the family name.

This is a valid theme.json snippet:

"typography": {
    "fontFamilies": [
        {
            "fontFamily": "SpaceMono, serif",
            "name": "SpaceMono",
            "fontFace": [
                {
                    "fontDisplay": "block",
                    "fontFamily": "SpaceMono",
                    "fontWeight": "400",
                    "fontStyle": "normal",
                    "fontStretch": "normal",
                    "src": [
                        "file:./assets/fonts/SpaceMono-Regular.ttf"
                    ]
                }
            ]
        }
    ]

As you can see in the theme.json schema the slug key is not required but this change makes it required because it is being accessed without checking if it's there and without providing an fallback value when is not. If you use a theme.json like that, this change produces 2 PHP warnings:

Warning:  Undefined array key "slug" in /projects/gutenberg/lib/experimental/register-webfonts-from-theme-json.php on line 121
Warning:  array_flip(): Can only flip string and integer values. Entry skipped in /projects/gutenberg/lib/experimental/register-webfonts-from-theme-json.php on line 127

Before seeing this, I proposed an alternative fix yesterday. Sorry for the partial overlap! thanks again for this valuable work! I would greatly appreciate it if you could review and test this PR.

[enodekciw]

Oh, didn't know that slug is not required, thanks for pointing this out!

Yup, saw your PR yesterday. Actually, I think that is an even better way to solve this issue.