fix: model validator assignment consistency

Fix inconsistency where model-level before and wrap validators received
different states during field assignment validation.

During assignment with validate_assignment=True:
- Before: before validators received dict with NEW field value
- After: before validators receive dict with OLD field value (consistent with wrap validators)

Both validator modes now consistently see the pre-update state during assignment.

Fixes: pydantic/pydantic#11823"

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