Refactor summarize diff implementation to use abstraction layer

Background

We have abstraction layer (https://docs.gitlab.com/ee/development/ai_features.html#how-to-implement-a-new-action) available for product teams to use to implement AI features.

As of this writing, the implementation of summarize diff feature isn't utilizing this and using its own pattern.

This will involve deprecating the old worker too 👍

Proposal

Follow https://docs.gitlab.com/ee/development/ai_features.html#how-to-implement-a-new-action without exposing it as an AiAction because we are automatically generating summaries so we don't want it exposed on the GraphQL API.