Fix server island script breaking when charset is added to content-type

[louisescher]

Changes

• This PR fixes an issue where the client-side script injected for server islands would fail if the content-type header was modified from an external source, for example a reverse proxy.

Testing

• Added a new server island to the server islands tests which sets a header with a media type. The test checks if the island gets inserted even with the media type present (content-type: text/html;charset=utf-8). Normal cases are automatically handled by the other server islands which do not set the header and instead have the default header (content-type: text/html)

Docs

No docs needed as this is a bug fix and doesn't change the default behavior.

[changeset-bot]

🦋 Changeset detected

Latest commit: 953b4dc

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[codspeed-hq]

CodSpeed Performance Report

Merging #12810 will not alter performance

_{Comparing louisescher:fix-server-island-content-type-check (953b4dc) with main (98f9e83)}

Summary

✅ 4 untouched benchmarks

[ematipico]

Thank you! This fix looks good to me