Introduce __validate__: User feedback for lf.query on autofix.

Example:

```
 class Foo(pg.Object):
      x: int

      def __validate__(self):
        # This triggers autofix.
        if self.x > 1:
          raise ValueError('value should be less or equal than 1.')

        # This triggers rule-based fix (user modification)
        if self.x < 0:
          self.rebind(x=0, skip_notification=True)

   foo = lf.query('Generate a Foo object', Foo, lm=lm, autofix=2)
   assert foo.x in (0, 1)
```

PiperOrigin-RevId: 672124993

langfun/core/coding/python/correction.py

@@ -81,23 +81,27 @@ def run_with_correction(
   # pylint: enable=g-import-not-at-top
 
   if max_attempts == 0:
-    result = execution.run(
-        code,
-        global_vars=global_vars,
-        sandbox=sandbox,
-        timeout=timeout,
-        outputs_intermediate=outputs_intermediate,
+    result = _maybe_custom_validate(
+        execution.run(
+            code,
+            global_vars=global_vars,
+            sandbox=sandbox,
+            timeout=timeout,
+            outputs_intermediate=outputs_intermediate,
+        )
     )
     return (result, code) if returns_code else result
 
   def result_and_error(code: str) -> tuple[Any, str | None]:
     try:
-      result = execution.run(
-          code,
-          global_vars=global_vars,
-          sandbox=sandbox,
-          timeout=timeout,
-          outputs_intermediate=outputs_intermediate,
+      result = _maybe_custom_validate(
+          execution.run(
+              code,
+              global_vars=global_vars,
+              sandbox=sandbox,
+              timeout=timeout,
+              outputs_intermediate=outputs_intermediate,
+          )
       )
       return (result, None)
     except Exception as e:  # pylint: disable=broad-exception-caught
@@ -190,3 +194,15 @@ def _error_feedback_str(error: Exception) -> str:
     )
   else:
     return f"Encountered {error.__class__.__name__}: {error}"
+
+
+def _maybe_custom_validate(result: Any) -> Any:
+  """Apply custom validation through __validate_generation__ method."""
+  if isinstance(result, dict) and "__result__" in result:
+    r = result["__result__"]
+  else:
+    r = result
+
+  if hasattr(r, "__validate__"):
+    r.__validate__()
+  return result

langfun/core/coding/python/correction_test.py

@@ -19,6 +19,7 @@
 from langfun.core.coding.python import correction
 from langfun.core.coding.python import errors
 from langfun.core.llms import fake
+import pyglove as pg
 
 
 class RunWithCorrectionTest(unittest.TestCase):
@@ -45,6 +46,32 @@ def test_run_with_correction(self):
     )
     self.assertEqual(result, 4)
 
+  def test_run_with_correction_upon_custom_validation(self):
+
+    class Foo(pg.Object):
+      x: int
+
+      def __validate__(self):
+        if self.x > 1:
+          raise ValueError('value should be less or equal than 1.')
+        if self.x < 0:
+          self.rebind(x=0, skip_notification=True)
+
+    result = correction.run_with_correction(
+        inspect.cleandoc("""
+            Foo(x=2)
+            """),
+        global_vars=dict(Foo=Foo),
+        lm=fake.StaticSequence([
+            inspect.cleandoc("""
+                CorrectedCode(
+                    corrected_code='Foo(x=-1)',
+                )
+                """),
+        ]),
+    )
+    self.assertEqual(result, Foo(0))
+
   def test_run_without_correction(self):
     result = correction.run_with_correction(
         inspect.cleandoc("""