fix: #3403329 Skip unknown image CAPTCHA font tokens instead of crashing

Jakob Prequested to merge
issue/captcha-3403329:3403329-post-drupal-10 into 2.x

Font tokens in image_captcha_fonts that no longer match an available font (for example after the module or font files moved during an upgrade) made _image_captcha_get_font_uri() return the whole font list, which was then passed to realpath() and caused a TypeError (HTTP 500) on the image route.

printString() now resolves the configured tokens once, skips unknown ones and falls back to the built-in font when none are left.

Closes #3403329

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