feat(registry): add requireModelPrefix to disable bare model ids

When the flag is true, a model resolves only as provider/model or as a
modelAliases id. A bare id returns 404 unknown model. GET /v1/models
lists every model in provider/model form and lists no bare ids.

The flag defaults to false, so existing deployments keep the sorted-first
bare-id collision winner.

llmgateway.go

@@ -55,6 +55,11 @@ type Config struct {
 	// (validateModelAliases, registry.go).
 	ModelAliases map[string]string          `json:"modelAliases,omitempty"`
 	Providers    map[string]*ProviderConfig `json:"providers,omitempty"`
+	// RequireModelPrefix, when true, makes every model addressable only as
+	// "provider/model" or as a ModelAliases id: a bare discovered id is not
+	// resolved (404 unknown model) and is not listed by GET /v1/models. It
+	// removes the sorted-first bare-id collision winner. Defaults to false.
+	RequireModelPrefix bool `json:"requireModelPrefix,omitempty"`
 	// ModelMeta declares per-model metadata overrides (feature v0.23):
 	// context window size and per-token cost, keyed by an exact
 	// "provider/model" id or a bare model id (applying wherever that bare