Enable Duo Chat for self-managed

config/feature_flags/ops/ai_duo_chat_switch.yml

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

ee/app/policies/ee/global_policy.rb

ee/app/policies/ee/group_policy.rb

ee/app/policies/ee/project_policy.rb

ee/app/services/llm/chat_service.rb

ee/app/services/llm/internal/completion_service.rb

ee/config/saas_features/duo_chat_on_saas.yml

ee/lib/ee/gitlab/saas.rb

ee/lib/gitlab/llm/ai_message.rb

ee/lib/gitlab/llm/chain/utils/authorizer.rb

ee/lib/gitlab/llm/chat_message.rb

ee/lib/gitlab/llm/tanuki_bot.rb

ee/spec/features/tanuki_bot_chat_spec.rb

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

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

...c/lib/gitlab/llm/chain/tools/json_reader/executor_spec.rb

ee/spec/lib/gitlab/llm/chain/utils/authorizer_spec.rb

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

...b/gitlab/llm/completions/summarize_all_open_notes_spec.rb