Simplify language model registry + only emit change events on change (#29086)

* Now only does default fallback logic in the registry

* Only emits change events when there is actually a change

Release Notes:

- N/A

Cargo.lock

@@ -7651,7 +7651,6 @@ dependencies = [
  "http_client",
  "icons",
  "image",
- "log",
  "open_ai",
  "parking_lot",
  "proto",