Astro content collections: link one phrase of a plain-string frontmatter field with a { text, href } sibling

A blog's coverCaption was z.string() rendered as text, so a photo credit couldn't be a link. Instead of switching the field to Markdown/HTML (needs set:html and sanitizing), add a sibling coverCaptionLink: z.object({ text, href: z.string().url() }).optional() and in the page split the caption at caption.indexOf(link.text): render slice before, <a>text</a>, slice after; fall back to plain text if the phrase isn't found. Same pattern as an existing titleLink field, so the frontmatter stays plain text for cards and previews. Note: z.string().url() now triggers a ts(6385) deprecation warning in astro check (0 errors).