Fixed CVE-2025-64459 -- Prevented SQL injections in Q/QuerySet via the _connector kwarg.

Thanks cyberstan for the report, Sarah Boyce, Adam Johnson, Simon
Charette, and Jake Howard for the reviews.

tests/queries/test_q.py

@@ -272,6 +272,11 @@ class QTests(SimpleTestCase):
                     Q(*items, _connector=connector),
                 )
 
+    def test_connector_validation(self):
+        msg = f"_connector must be one of {Q.AND!r}, {Q.OR!r}, {Q.XOR!r}, or None."
+        with self.assertRaisesMessage(ValueError, msg):
+            Q(_connector="evil")
+
     def test_referenced_base_fields(self):
         # Make sure Q.referenced_base_fields retrieves all base fields from
         # both filters and F expressions.