Add validate_schema_with config method for structured JSON output

[LasmarKhalifa]

Follow-up to #1007. Once structured JSON output exists, add a config method that designates a secondary model to validate a cog's structured output.

This would be used to validate that the output conforms to the user's needs. For example, it could flag that a field was not populated although the prompt indicated that it was supposed to be always present.

an example of usage would be

config do
  chat(:extract) do
    validate_schema_with "claude-opus-4-8"
  end
end