Shift some validations to BuiltInTool

What does this MR do and why?

This MR shifts some validation responsibility from ee/spec/lib/ai/catalog/built_in_tool_definitions_spec.rb to the BuiltInTool model.

Note: BuiltInTool uses ActiveRecord::FixedItemsModel::Model to wrap a static list of data (which is in Ai::Catalog::BuiltInToolDefinitions). As such, it is not an ActiveRecord model, but just a PORO that is wrapping a list of 47 Hash items.

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