test(backend): add tests for upstream model tracking and model source filtering

Cover IsValidModelSource/NormalizeModelSource, resolveModelDimensionExpression SQL expressions, invalid model_source 400 responses on both GetModelStats and GetUserBreakdown, upstream_model in scan/insert SQL mock expectations, and updated passthrough/billing test signatures.

backend/internal/repository/usage_log_repo_breakdown_test.go

@@ -5,6 +5,7 @@ package repository
 import (
 	"testing"
 
+	"github.com/Wei-Shaw/sub2api/internal/pkg/usagestats"
 	"github.com/stretchr/testify/require"
 )
 
@@ -16,8 +17,8 @@ func TestResolveEndpointColumn(t *testing.T) {
 		{"inbound", "ul.inbound_endpoint"},
 		{"upstream", "ul.upstream_endpoint"},
 		{"path", "ul.inbound_endpoint || ' -> ' || ul.upstream_endpoint"},
-		{"", "ul.inbound_endpoint"},           // default
-		{"unknown", "ul.inbound_endpoint"},     // fallback
+		{"", "ul.inbound_endpoint"},        // default
+		{"unknown", "ul.inbound_endpoint"}, // fallback
 	}
 
 	for _, tc := range tests {
@@ -27,3 +28,23 @@ func TestResolveEndpointColumn(t *testing.T) {
 		})
 	}
 }
+
+func TestResolveModelDimensionExpression(t *testing.T) {
+	tests := []struct {
+		modelType string
+		want      string
+	}{
+		{usagestats.ModelSourceRequested, "model"},
+		{usagestats.ModelSourceUpstream, "COALESCE(NULLIF(TRIM(upstream_model), ''), model)"},
+		{usagestats.ModelSourceMapping, "(model || ' -> ' || COALESCE(NULLIF(TRIM(upstream_model), ''), model))"},
+		{"", "model"},
+		{"invalid", "model"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.modelType, func(t *testing.T) {
+			got := resolveModelDimensionExpression(tc.modelType)
+			require.Equal(t, tc.want, got)
+		})
+	}
+}