Change content schemas to require alt text with an image

In our content schemas, it is possible to provide an featured image but no alt text for it. Our schemas hold them separately as in:

featuredImage: image().optional(),
featuredImageAlt: z.string().optional(),

a schema like this would prevent authors from adding an image without also including alt text:

featuredImage: z.object({
    src: image(),
    alt: z.string()
}).optional(),

which also makes it possible to provide a default with accurate alt text, as in:

featuredImage: z.object({
    src: image(),
    alt: z.string()
}).optional().default({
  src: PlaceholderImage
  alt: 'the p5 logo in a grey box'
}),

Start by locating the content schema definitions described in the issue and inspect how featuredImage and featuredImageAlt are consumed. Update the schema so an image always has accompanying alt text, including any default featured image, then verify that existing content validates and images without alt text are rejected.