Draft: Fix Syntax Bug in AiGatewayCodeGenerationMessage

What does this MR do and why?

While running code generation evaluations from prompt-lib, I kept getting the following error:

Request failed for POST http://gdk.test:3000/api/v4/code_suggestions/completions 422 {"detail":[{"type":"string_type","loc":["body",2,"prompt"],"msg":"Input should be a valid string","input":{"trigger_type":"comment","instruction":""}}]})

I traced it down to this line here.

def prompt
  params[:instruction] || DEFAULT_INSTRUCTION
end

params[:instruction] is an object, but prompt() is expected to return a string. Returning the instruction value of the instruction object fixes this issue.

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