language_models: Use weak entities in subscribe/observes around the language model registry (#52312)

## Context

I was getting some leak detection failures in evals and tracked it down
to these entities getting passed into observe/subscribe callbacks and
causing cycles.

Release Notes:

- N/A

Co-authored-by: Lukas Wirth <me@lukaswirth.dev>
b28990e471Ben Brandt committed on 3/24/2026, 11:08:44 AM· committed by GitHubparent7c18520
2 files changedLine totals unavailable