[proxy] Exempt non-inference endpoints from the model allowlist gate

GET /v1/models carries no model, and management's per-model allowlist
fails closed on an undetermined one, so gateway model discovery denied
with model_blocked for every account that enables a model allowlist. The
client treats a failed discovery as silent and falls back to its built-in
list, so the operator sees an empty picker with no error to chase.

The router already classifies these paths and authorises the route against
the caller's groups before allowing them, so mark them non-inference there
and let the limits gate skip a pre-flight that has no model to evaluate
and no tokens to book. The marker comes from the router's own path
classification, never from client input, so an inference request cannot
set it to escape the allowlist.

proxy/internal/middleware/builtin/llm_router/middleware.go

@@ -109,6 +109,7 @@ func (m *Middleware) MetadataKeys() []string {
 		middleware.KeyLLMAuthorisingGroups,
 		middleware.KeyLLMPolicyDecision,
 		middleware.KeyLLMPolicyReason,
+		middleware.KeyLLMNonInference,
 	}
 }
 
@@ -193,7 +194,9 @@ func (m *Middleware) Invoke(_ context.Context, in *middleware.Input) (*middlewar
 		route, outcome := m.matchModelless(requestPath(in.URL), in.UserGroups)
 		switch outcome {
 		case matchOutcomeFound:
-			return m.allowWithRoute(route, in.UserGroups), nil
+			out := m.allowWithRoute(route, in.UserGroups)
+			out.Metadata = append(out.Metadata, middleware.KV{Key: middleware.KeyLLMNonInference, Value: "true"})
+			return out, nil
 		case matchOutcomeUnauthorised:
 			// A recognised model-less endpoint exists but no provider
 			// authorises the caller — deny as an authorisation failure

proxy/internal/middleware/builtin/llm_router/middleware_test.go

@@ -60,6 +60,7 @@ func TestMiddlewareIdentity(t *testing.T) {
 		[]string{
 			middleware.KeyLLMResolvedProviderID,
 			middleware.KeyLLMAuthorisingGroups,
+			middleware.KeyLLMNonInference,
 			middleware.KeyLLMPolicyDecision,
 			middleware.KeyLLMPolicyReason,
 		},
@@ -197,6 +198,12 @@ func TestRouter_ModelLessPath_RoutesToAuthorisedProvider(t *testing.T) {
 
 	provider, _ := metaValue(t, out.Metadata, middleware.KeyLLMResolvedProviderID)
 	assert.Equal(t, "openai-prod", provider, "resolved provider must be the authorised route")
+
+	// The limits gate reads this to tell "no model applies here" from
+	// "the model could not be determined", which fails closed.
+	nonInference, ok := metaValue(t, out.Metadata, middleware.KeyLLMNonInference)
+	require.True(t, ok, "model-less allow must mark the request non-inference")
+	assert.Equal(t, "true", nonInference)
 }
 
 func TestRouter_ModelLessPath_MultiProviderDeclarationOrder(t *testing.T) {