Add preferred language to GET /user output

What does this MR do and why?

Add preferred language to GET /user output

This change exposes a current user's preferred language in the GET /user endpoint. It also allows ai_workflow scope to receive current_user information.

References

#548506 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Invoke the class to generate an oauth token with the ai_workflow scope

   Ai::DuoWorkflows::CreateOauthAccessTokenService.new(current_user: User.first).execute

2. This token can be used to authenticate various endpoints. For example:

    curl --header "Authorization: Bearer $TOKEN_FROM_LAST_STEP" --url "https://gdk.test/api/v4/user"

3. Ensure that preferred_language data is exposed

MR acceptance checklist

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