Use selected text in base prompt if present

What does this MR do and why?

When user selects some code, it makes sense to include this in the prompt (and prefer over blob from referrer).

Related to #431359 (closed)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

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

1. In rails console enable code_tasks

   Feature.enable(:code_tasks)

2. Open gitlab duo chat UI
3. In a separate browser tab open graphql-explorer and send message request below:

mutation {
  aiAction(input: {chat: {content: "Refactor selected code", currentFile: {
    fileName: "test.py",
    selectedText: "
def extract
  lines = content.to_s.lines
  comment_block = []
  trimmed_lines = 0
end
",
    contentAboveCursor: "
module CodeSuggestions
  class InstructionsExtractor
    EMPTY_LINES_LIMIT = 1
    MIN_LINES_OF_CODE = 5
",
  }}}) {
    clientMutationId
    errors
  }
}

1. check in chat UI that an answer relevant to the selected text is returned

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.