* Add a new mutation summarizeNewMergeRequest to the GraphQL API.
* Add a new service Llm::SummarizeNewMergeRequestService to handle the mutation.
* Add a new template Gitlab::Llm::Templates::SummarizeNewMergeRequest to generate the prompt for the AI model.
* Add a new completions worker Gitlab::Llm::VertexAi::Completions::SummarizeNewMergeRequest to handle the completion request.
* Add a new response modifier Gitlab::Llm::VertexAi::ResponseModifiers::Predictions to modify the response from the AI model.
* Add a new subscription Gitlab::Llm::GraphqlSubscriptionResponseService to publish the response from the AI model to the GraphQL subscription.
* Add a new policy ProjectPolicy#summarize_new_merge_request to control access to the new feature.
* Add a new feature flag add_ai_summary_for_new_mr to control the availability of the new feature.
* Add a new documentation page to explain the new feature.

EE: true