Stream summarized notes

What does this MR do and why?

This enables streaming of summarized notes and provides the capabilities to render the streamed chunks on the frontend.

Streaming only works with Anthropic and the summarize_notes_with_anthropic feature flag being turned on. (currently turned off in production).

The FE change is backwards compatible during deployment as we already pass through the clientSubscriptionId from the mutation to the broadcasting of the subscription.

Screenshots or screen recordings

streamed_notes_demo

How to set up and validate locally

1. Enable AI features: https://docs.gitlab.com/ee/development/ai_features.html
2. Enable the feature flag: Feature.enable(:summarize_notes_with_anthropic)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.