fix: replace :first-child with :first-of-type in mapbox-gl.css (fixes #13639)

[ragini-pandey]

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page.
• Write tests for all new functionality and make sure the CI checks pass.
• Document any changes to public APIs.
• Post benchmark scores if the change could affect performance.
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes.
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port.
• Tag @mapbox/gl-native if this PR disables any test because it also needs to be disabled on their side.
• Create a ticket for gl-native to groom in the MAPSNAT JIRA queue if this PR includes shader changes or features not present in the native side or if it disables a test that's not disabled there.

Description

Fixes #13639.

Emotion (a popular CSS-in-JS library) warns at runtime when it encounters CSS rules using the :first-child pseudo-class, because that selector is not safe for server-side rendering (SSR):

The pseudo-class ":first-child" is potentially unsafe when doing server-side rendering. Try changing it to ":first-of-type".

Change

In src/css/mapbox-gl.css, the single occurrence of :first-child:

/* before */
.mapboxgl-ctrl-group button:first-child {
    border-radius: 4px 4px 0 0;
}

was replaced with :first-of-type:

/* after */
.mapboxgl-ctrl-group button:first-of-type {
    border-radius: 4px 4px 0 0;
}

Why this is safe

All direct children of .mapboxgl-ctrl-group are <button> elements, so :first-child and :first-of-type are semantically equivalent in this context. The change silences the Emotion SSR warning without any visual or behavioural difference.

[CLAassistant]

All committers have signed the CLA.