Preserve recursively-set value in defaultdict.__missing__ (#7718)

CPython's defaultdict.__missing__ (Modules/_collectionsmodule.c::defdict_missing)
calls default_factory() first; if the factory's recursion already populated
self[key] while running, the existing value is preserved instead of being
overwritten.

RustPython ships a Python fallback at Lib/collections/_defaultdict.py
(the C _collections.defaultdict is not available). That fallback
unconditionally executed self[key] = val after the factory returned,
overwriting any value the recursive call had already stored.

Add a 'if key in self: return self[key]' guard before the assignment.
dict.__contains__ does not invoke __missing__, so there's no recursion
risk; in the common non-reentrant case the check is False and behavior
is unchanged.

Unmasks test_defaultdict.TestDefaultDict.test_factory_conflict_with_set_value.

Lib/collections/_defaultdict.py

@@ -17,6 +17,10 @@ class defaultdict(dict):
             val = self.default_factory()
         else:
             raise KeyError(key)
+        # CPython parity: a recursive __missing__ via factory() may have
+        # already populated key; preserve that value instead of overwriting.
+        if key in self:
+            return self[key]
         self[key] = val
         return val
 

Lib/test/test_defaultdict.py

@@ -186,7 +186,6 @@ class TestDefaultDict(unittest.TestCase):
         with self.assertRaises(TypeError):
             i |= None
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; AssertionError: 1 != 2
     def test_factory_conflict_with_set_value(self):
         key = "conflict_test"
         count = 0