program.py: route from_bytes_backrefs through deser_auto

The previous implementation went through CLVMTree.from_bytes → deserialize_as_tuples,
which is a classic-format-only parser. As a result, Program.from_bytes_backrefs()
raised ValueError on any blob using the 0xfe backref opcode, despite the method's
name implying backref support. After this change:
  - still rejects the serde_2026 magic prefix up front
  - delegates to deser_auto (Rust binding), which handles both classic and backref formats
  - adds regression test that round-trips a tree-with-shared-subtrees through
    backref encoder and from_bytes_backrefs (asserting 0xfe presence)

Fixes Cursor bot finding on PR #708.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: richardkiss

wheel/python/clvm_rs/program.py

@@ -51,8 +51,7 @@ def from_bytes_backrefs(cls, blob: bytes) -> Program:
         """Deserialize classic or backrefs format only (rejects serde_2026)."""
         if blob.startswith(SERDE_2026_MAGIC_PREFIX):
             raise ValueError("unexpected serde_2026 format; use from_bytes() for auto-detection")
-        obj, cursor = cls.from_bytes_with_cursor(blob, 0)
-        return obj
+        return cls.wrap(deser_auto(blob))
 
     @classmethod
     def from_bytes_with_cursor(

wheel/python/tests/test_serialize.py

@@ -140,7 +140,7 @@ def test_repr_clvm_tree(self):
         with self.assertRaises(ValueError):
             Program.fromhex("ff8085")
 
-        o = Program.from_bytes_backrefs(bytes.fromhex("ff808185"))
+        o, _ = Program.from_bytes_with_cursor(bytes.fromhex("ff808185"), 0)
         self.assertEqual(repr(o._unwrapped_pair[0]), "<CLVMTree: 80>")
         self.assertEqual(repr(o._unwrapped_pair[1]), "<CLVMTree: 8185>")
 
@@ -179,6 +179,24 @@ def test_backrefs_parser_rejects_2026(self):
         with self.assertRaises(ValueError):
             Program.from_bytes_backrefs(prefixed)
 
+    def test_from_bytes_backrefs_with_actual_backrefs(self):
+        """Test that from_bytes_backrefs actually parses backrefs format (0xfe opcodes)."""
+        # Construct a tree with shared subtrees that will trigger backref encoding
+        shared = Program.to([b"shared_atom", b"another_shared_atom"])
+        p = Program.to([shared, shared, shared, shared, shared])
+        
+        # Serialize using backrefs format
+        blob = serialize(deserialize(bytes(p), "legacy"), "backrefs")
+        
+        # Assert the blob contains at least one 0xfe backref opcode
+        self.assertIn(0xfe, blob, "backrefs-encoded blob should contain at least one 0xfe byte")
+        
+        # Round-trip through from_bytes_backrefs
+        p2 = Program.from_bytes_backrefs(blob)
+        
+        # Verify equality (by tree hash)
+        self.assertEqual(p, p2)
+
 
 class ClvmTreeToLazyNodeTest(unittest.TestCase):
     """Tests for clvm_tree_to_lazy_node."""