Skip overridden items for empty dictionaries in JSON encoding (#9018)

Split the Python 3.14-compatible empty native dictionary fast path from #8954. Extend the existing JSON snippet with root, nested, sorting, and nonempty subclass regression coverage.

Assisted-by: OpenAI Codex:model identifier unavailable

crates/stdlib/src/json.rs

@@ -1160,6 +1160,10 @@ mod _json {
             out: &mut Wtf8Buf,
             vm: &VirtualMachine,
         ) -> PyResult<()> {
+            if dict.is_empty() {
+                out.push_str("{}");
+                return Ok(());
+            }
             let mut items = self.dict_items(obj, dict, vm)?;
             if items.is_empty() {
                 out.push_str("{}");

extra_tests/snippets/stdlib_json.py

@@ -67,6 +67,24 @@ assert "{}" == json.dumps({})
 assert "{}" == json_dump({})
 assert round_trip_test({"a": "b"})
 
+
+class EmptyDictWithItems(dict):
+    def items(self):
+        raise AssertionError("items() must not be called for an empty dictionary")
+
+
+for sort_keys in (False, True):
+    assert json.dumps(EmptyDictWithItems(), sort_keys=sort_keys) == "{}"
+    assert json.dumps([EmptyDictWithItems()], sort_keys=sort_keys) == "[{}]"
+
+
+class NonemptyDictWithItems(dict):
+    def items(self):
+        return [("replacement", 2)]
+
+
+assert json.dumps(NonemptyDictWithItems(original=1)) == '{"replacement": 2}'
+
 # should reject non-str keys in jsons
 assert_raises(json.JSONDecodeError, lambda: json.loads('{3: "abc"}'))
 assert_raises(json.JSONDecodeError, lambda: json_load('{3: "abc"}'))