Add /tests slash command

What does this MR do and why?

Adds a new tool for writing tests. This tool also contains a slash command /tests (then this tool is used directly).

Related to #431578 (closed) (Task)

Related to #429915 (closed) (Parent Issue)

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Feature.enable(:slash_commands)

2. Open chat UI in one tab and in another tab send a graphql query below:

mutation {
  aiAction(input: {chat: {content: "/tests", currentFile: {
    selectedText: "def hello_world\nputs 'Hello, World'\nend",
    fileName: "hello.rb"
  }}}) {
    clientMutationId
    errors
  }
}

1. Check the response in chat UI

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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