Add the tanuki_bot migration/model

doc/api/graphql/reference/index.md

ee/app/controllers/llm/tanuki_bot_controller.rb

ee/app/graphql/mutations/ai/action.rb

ee/app/graphql/types/ai/base_method_input_type.rb

ee/app/graphql/types/ai/tanuki_bot_input_type.rb

ee/app/models/ee/user.rb

ee/app/services/llm/base_service.rb

ee/app/services/llm/execute_method_service.rb

ee/app/services/llm/tanuki_bot_service.rb

ee/lib/gitlab/llm/open_ai/completions/factory.rb

ee/lib/gitlab/llm/open_ai/completions/tanuki_bot.rb

ee/lib/gitlab/llm/open_ai/response_modifiers/tanuki_bot.rb

ee/lib/gitlab/llm/open_ai/templates/tanuki_bot.rb

ee/lib/gitlab/llm/tanuki_bot.rb

ee/spec/graphql/mutations/ai/action_spec.rb

...pec/lib/gitlab/llm/open_ai/completions/tanuki_bot_spec.rb

.../gitlab/llm/open_ai/response_modifiers/tanuki_bot_spec.rb

ee/spec/lib/gitlab/llm/tanuki_bot_spec.rb

ee/spec/models/ee/user_spec.rb

ee/spec/services/llm/base_service_spec.rb