Raise exception when parameter_source set for a non-dangling build

The following discussion from !34706 (merged) should be addressed:

• @dosuken123 started a discussion: (+2 comments)

  Should we also validate the command.source here? I don't think we want to specify the free-style content for the other event types such as merge_request_event.

  For example,

  def content
    next unless command.content.present?
    next unless command.dangling_builds?
  
    command.content
  end

  and

  def dangling_builds?
    %i[webide ondemand_dast_scan].include?(source)
  end