injectMessages

Subscribe to a scoped messages stream.

Contract:

• At the root (no target) this returns stream.messages — the always-on root projection; no extra subscription is opened.
• For any non-root namespace, call-time triggers a ref-counted messages subscription scoped to that namespace. The subscription is released automatically when the calling component / service is destroyed (and the registry closes the underlying server subscription when the last consumer leaves).

Messages are always BaseMessage class instances from @langchain/core/messages.