Rename `Gitlab::Llm::TanukiBot`

Context

Before the name "GitLab Duo" existed, we called Duo Chat "Tanuki Bot". As a result, there is a lot of code in gitlab-org/gitlab that references this old name.

Definition of done

Gitlab::Llm::TanukiBot class is more related to Embeddings than to Duo Chat as a whole.

It may make more sense for the contents of this class to live at ee/app/services/llm/generate_embeddings_completions_service and to call it GenerateEmbeddingsCompletionsService because that is what it is really doing in the execute method. Also open to other name ideas!

All unit tests for Llm::TanukiBot would also need to be moved and updated.

There are 2 other methods in the class called self.enabled_for and self.show_breadcrumbs_entry_point?. These methods are really helper methods used in views. Those methods can stay in ee/lib/gitlab/llm/tanuki_bot.rb for now or you can move them into a new class in ee/helpers because that is where helper methods should live in Rails.