Fixed querystring search query type

[pnicolli]

Current query validation is

const query = z.object({
  i: z.string(),
  o: z.string(),
  v: z.array(z.string()),
});

But at least one index (SearchableText), but I assume many, actually require a literal string as v instead of an array of strings.

Yes this could be a lot more specific, this could specify the right v type for each index or something similar, but before embarking on that journey I think we can start simple and just allow plain strings.

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📚 Documentation preview 📚: https://volto--6034.org.readthedocs.build/

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