fix(bn256/gnark): fix inverted TestBytes logic in G1/G2 Unmarshal

The TestBytes function returns true when all bytes are zero (point at infinity).
The condition was inverted, causing non-zero points to be treated as infinity.

bn256/gnark/g1.go

@@ -44,8 +44,8 @@ func (g *G1) Unmarshal(buf []byte) (int, error) {
 		return 0, errors.New("invalid G1 point size")
 	}
 
-	if !bitutil.TestBytes(buf[:64]) {
-		// point at infinity
+	if bitutil.TestBytes(buf[:64]) {
+		// point at infinity (all zeros)
 		g.inner.X.SetZero()
 		g.inner.Y.SetZero()
 		return 64, nil

bn256/gnark/g2.go

@@ -32,8 +32,8 @@ func (g *G2) Unmarshal(buf []byte) (int, error) {
 		return 0, errors.New("invalid G2 point size")
 	}
 
-	if !bitutil.TestBytes(buf[:128]) {
-		// point at infinity
+	if bitutil.TestBytes(buf[:128]) {
+		// point at infinity (all zeros)
 		g.inner.X.A0.SetZero()
 		g.inner.X.A1.SetZero()
 		g.inner.Y.A0.SetZero()

bn256/gnark/native_format_test.go

@@ -28,10 +28,14 @@ func TestSerRoundTrip(t *testing.T) {
 	bytesG2 := expectedG2.Marshal()
 
 	var gotG1 G1
-	gotG1.Unmarshal(bytesG1)
+	if _, err := gotG1.Unmarshal(bytesG1); err != nil {
+		t.Fatalf("G1 unmarshal failed: %v", err)
+	}
 
 	var gotG2 G2
-	gotG2.Unmarshal(bytesG2)
+	if _, err := gotG2.Unmarshal(bytesG2); err != nil {
+		t.Fatalf("G2 unmarshal failed: %v", err)
+	}
 
 	if !expectedG1.inner.Equal(&gotG1.inner) {
 		t.Errorf("serialization roundtrip failed for G1")