fix: prevent RecursionError when a dict contains itself (#592)

BaseDict._get_dict_or_value() recursively unwraps nested benedict
values with no cycle detection, so assigning a benedict into its own
nested descendant a second time recurses forever and crashes with
RecursionError. Track ids of mappings currently being unwrapped and
raise a clear ValueError as soon as a self-reference is detected,
instead of recursing into unrelated, equally unprotected traversal
code until the interpreter stack overflows.

tests/github/test_issue_0592.py

@@ -0,0 +1,44 @@
+import unittest
+
+from benedict import benedict
+
+
+class github_issue_0592_test_case(unittest.TestCase):
+    """
+    This class describes a github issue 0592 test case.
+    https://github.com/fabiocaccamo/python-benedict/issues/592
+
+    To run this specific test:
+    - Run python -m unittest tests.github.test_issue_0592
+    """
+
+    def test_assigning_dict_to_its_own_nested_descendant_does_not_raise(self) -> None:
+        # not self-referential yet at assignment time, so it must keep working
+        d = benedict(keyattr_enabled=True, keyattr_dynamic=True)
+        d.a.b.c = d
+        self.assertIs(dict.__getitem__(d, "a")["b"]["c"], d)
+
+    def test_assigning_dict_to_its_own_nested_descendant_twice_does_not_recurse(
+        self,
+    ) -> None:
+        # once `d` contains itself, unwrapping it again must not raise
+        # RecursionError (it used to crash the process)
+        d = benedict(keyattr_enabled=True, keyattr_dynamic=True)
+        d.a.b.c = d
+        with self.assertRaises(ValueError):
+            d.a.b.d.e = d
+
+    def test_self_assignment_at_new_key_still_works(self) -> None:
+        # assigning a dict to itself under a new key is unaffected by the fix
+        d = benedict({"a": {"b": 1}})
+        d["self"] = d
+        self.assertIsInstance(d["self"], benedict)
+        self.assertEqual(d["self"]["a"], {"b": 1})
+
+    def test_normal_nested_dict_assignment_is_unaffected(self) -> None:
+        # non self-referential values must still be unwrapped as before
+        inner = benedict({"x": 1})
+        outer = benedict()
+        outer["inner"] = inner
+        self.assertEqual(outer, {"inner": {"x": 1}})
+        self.assertIsNot(outer["inner"], inner)