Keeps three invariants across #116156 (@fangliquanflq) and #116292 (@Finn763):
a configured providers.llamacpp entry wins over managed-server detection on the
runtime path; an external llama-server on a local_runtime.detect_ports port is
what /model > Local (switch_model with the picker's provider id) resolves to;
a staged GGUF alone keeps the picker's id resolvable so the runtime seam, not
the provider gate, reports a missing server.
Drops the endpoint.py config auto-load from #116292: both production callers of
resolve_llamacpp_endpoint() now pass the loaded config (#116156), so loading it
again inside the resolver was defense-in-depth. Drops the change-detector test
on the forwarded config object and the redundant negative control.
Docs: local-models page now shows detect_ports and the providers.llamacpp
override for a llama-server the user runs on a fixed port.
34fb4ab482