API for fetching async AI responses

doc/api/graphql/reference/index.md

ee/app/graphql/ee/types/query_type.rb

ee/app/graphql/resolvers/ai/messages_resolver.rb

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

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

ee/app/services/llm/base_service.rb

ee/lib/gitlab/llm/cache.rb

ee/lib/gitlab/llm/cached_message.rb

ee/lib/gitlab/llm/graphql_subscription_response_service.rb

ee/spec/graphql/types/query_type_spec.rb

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

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

.../gitlab/llm/graphql_subscription_response_service_spec.rb

...gitlab/llm/open_ai/completions/generate_test_file_spec.rb

ee/spec/requests/api/graphql/ai_messages_spec.rb

...ed_examples/services/llm/async_service_shared_examples.rb