fix: define "after" model validator correctly

https://docs.pydantic.dev/latest/concepts/validators/#model-validators
says that "after" model validators are instance methods and should
return the validated instance.  The previous test code produced a
warning from pydantic >= 2.9.0:

```
UserWarning: A custom validator is returning a value other than `self`.
Returning anything other than `self` from a top level model validator isn't supported when validating via `__init__`.
See the `model_validator` docs (https://docs.pydantic.dev/latest/concepts/validators/#model-validators) for more details.
```

Author: cjwatson

tests/test_decorators.py

@@ -120,9 +120,9 @@ def _validate_x_before(cls, v):
             return v
 
         @model_validator(mode="after")
-        def _validate_x_after(cls, v):
-            mock_after(v)
-            return v
+        def _validate_x_after(self):
+            mock_after()
+            return self
 
         # this also needs to work
         @model_validator(mode="after")
@@ -133,6 +133,6 @@ def _validate_x_after_cm(cls, v):
 
     m = Model(x="2")
     mock_before.assert_called_once_with({"x": "2"})
-    mock_after.assert_called_once_with(m)
+    mock_after.assert_called_once_with()
     mock_after_cm.assert_called_once_with(m)
     assert m.x == 2