fix(registry): log model-id collisions at WARN, not ERROR

A model id served by two configured providers is resolved
deterministically by the registry, the winner is named in the message,
and no request is affected. Logging it at ERROR put 144 ERROR lines in a
healthy pod's boot log on a cluster with overlapping openai,
openai-audio, copilot and macstudio catalogs, which is exactly the noise
that hides a real failure.

Adds Gateway.warnf beside logf and errorf, and a warn field on
modelRegistry carrying it. The field is set by the production caller
after newModelRegistry returns rather than added as a fourth constructor
parameter: 45 call sites pass a logger today, almost all of them tests
passing a no-op, and threading a parameter through all of them would
bury a one-line severity change in unrelated churn. A nil warn falls
back to log, so a registry built without one keeps its current behaviour
instead of panicking or dropping the line.

registry.warnf takes an already-formatted string so both call shapes
stay single-variadic-argument. Yaegi v0.16.1's CFG builder panics on a
call to a struct FIELD of variadic func type with two or more variadic
arguments, which is the restriction the log field beside it already
documents.

Verified under the real interpreter, not only compiled: yaegi-check
drives a genuine two-provider collision and now prints WARN. The
severity test fails if the call site reverts to log, so the level cannot
drift back unnoticed.

Only this message moves. Genuine failures — discovery errors, metadata
capture panics, encoding failures — stay at ERROR.

llmgateway.go

@@ -588,6 +588,9 @@ func newGateway(ctx context.Context, next http.Handler, config *Config, name str
 	if err != nil {
 		return nil, err
 	}
+	// Routes the self-resolving conditions (a model id two providers both
+	// serve) to WARN instead of ERROR — see the registry's warn field.
+	registry.warn = g.warnf
 	// ctx is passed through unwrapped, not re-bounded to warmFillTimeout here:
 	// warmFill already gives each discovery-enabled provider its own fresh
 	// warmFillTimeout budget per provider (registry.go). Wrapping ctx to a