fix: ensure model before validators receive consistent pre-update state during assignment

Author: mot3

src/validators/model.rs

@@ -215,7 +215,6 @@ impl Validator for ModelValidator {
         if let Ok(old_extra) = model.getattr(intern!(py, DUNDER_MODEL_EXTRA_KEY))?.downcast::<PyDict>() {
             input_dict.update(old_extra.as_mapping())?;
         }
-        input_dict.set_item(field_name, field_value)?;
 
         let output = self
             .validator

tests/validators/test_model.py

@@ -1165,12 +1165,12 @@ class MyModel:
         (
             core_schema.with_info_before_validator_function,
             ({'b': 2}, 'ValidationInfo(config=None, context=None, data=None, field_name=None)'),
-            ({'a': 10, 'b': 2}, "ValidationInfo(config=None, context=None, data=None, field_name='a')"),
+            ({'a': 1, 'b': 2}, "ValidationInfo(config=None, context=None, data=None, field_name='a')"),
         ),
         (
             core_schema.with_info_wrap_validator_function,
             ({'b': 2}, 'ValidationInfo(config=None, context=None, data=None, field_name=None)'),
-            ({'a': 10, 'b': 2}, "ValidationInfo(config=None, context=None, data=None, field_name='a')"),
+            ({'a': 1, 'b': 2}, "ValidationInfo(config=None, context=None, data=None, field_name='a')"),
         ),
     ],
 )