Align anthropic prompts to AIGateway

What does this MR do and why?

See #427453 (closed)

AI Gateway expects to receive model_provider in order to decided which model provider to use. By default AI Gateway uses vertex models

It also puts <new_code> opening tag into Assistant: section of anthropic code generation response to steer model into right format, and also help stripping tags with stop sequence

Screenshots or screen recordings

I, [2023-10-05T14:07:08.976964 #65712]  INFO -- : jaccard similarity tri-grams: 0.554
I, [2023-10-05T14:07:08.977016 #65712]  INFO -- :
<<< expected response:

    def self.find_by_name(name)
      find_by(name: name)
    end
>>> real response:

def self.find_by_name(name)
  where(name: name).first
end


I, [2023-10-05T14:07:09.478315 #65712]  INFO -- : path: snippets/ruby/valid_name.rb, cursor: 1
I, [2023-10-05T14:07:12.058357 #65712]  INFO -- : duration: 2.5799930095672607
I, [2023-10-05T14:07:12.058501 #65712]  INFO -- : jaccard similarity tri-grams: 0.221
I, [2023-10-05T14:07:12.058511 #65712]  INFO -- :
<<< expected response:

  def valid_name?
    name.present? && name.length <= 255
  end
>>> real response:

def valid_name?
  name =~ /\A[a-zA-Z0-9_\-\.]*\z/
end

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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