Fix bug introduced in monkey patch of Dry::Schema

[visini]

We are not using fast-mcp directly in our project (only via a dependency), but we're using dry-validation / dry-schema quite a bit. Today, I noticed this bug introduced by fast-mcp (1.6.0) which is causing errors in our app (entirely unrelated to fast-mcp, since we don't use it, but it's apparently caused by the monkey patch in lib/mcp/tool.rb).

See the failing test case (which simulates our usage of Dry::Schema). The changes to lib/mcp/tool.rb make it pass and address the bug.

[josevalim]

For those running into this, it can be temporarily fixed by adding the following into an initializer (or config/environments/development.rb if using tidewave):

  Dry::Schema::Macros::Hash.class_eval do
    def call(*args, &block)
      if block
        # Use current context to track nested context if available
        context = MetadataContext.current
        if context
          context.with_nested(name) do
            original_call(*args, &block)
          end
        else
          original_call(*args, &block)
        end
      else
        original_call(*args)
      end
    end
  end