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/pkg/usagestats/usage_log_types_test.go

@@ -0,0 +1,47 @@
+package usagestats
+
+import "testing"
+
+func TestIsValidModelSource(t *testing.T) {
+	tests := []struct {
+		name   string
+		source string
+		want   bool
+	}{
+		{name: "requested", source: ModelSourceRequested, want: true},
+		{name: "upstream", source: ModelSourceUpstream, want: true},
+		{name: "mapping", source: ModelSourceMapping, want: true},
+		{name: "invalid", source: "foobar", want: false},
+		{name: "empty", source: "", want: false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := IsValidModelSource(tc.source); got != tc.want {
+				t.Fatalf("IsValidModelSource(%q)=%v want %v", tc.source, got, tc.want)
+			}
+		})
+	}
+}
+
+func TestNormalizeModelSource(t *testing.T) {
+	tests := []struct {
+		name   string
+		source string
+		want   string
+	}{
+		{name: "requested", source: ModelSourceRequested, want: ModelSourceRequested},
+		{name: "upstream", source: ModelSourceUpstream, want: ModelSourceUpstream},
+		{name: "mapping", source: ModelSourceMapping, want: ModelSourceMapping},
+		{name: "invalid falls back", source: "foobar", want: ModelSourceRequested},
+		{name: "empty falls back", source: "", want: ModelSourceRequested},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := NormalizeModelSource(tc.source); got != tc.want {
+				t.Fatalf("NormalizeModelSource(%q)=%q want %q", tc.source, got, tc.want)
+			}
+		})
+	}
+}