[pythonic config] properly propagate default values set at the top le…

…vel for nested config classes (#27206)

## Summary

Ensures that default values on Pythonic Config get properly propagated
to inner Config classes, e.g.

```python
class ANestedOpConfig(Config):
    an_int: int = 1
    a_bool: bool = True

class AnOpConfig(Config):
    a_string: str = "foo"
    a_nested: ANestedOpConfig = ANestedOpConfig(an_int=5)
```

An alternative impl which uses some utils in a stacked PR can be found
at #27209.

## Test Plan

New unit test; todo add a few more

python_modules/dagster/dagster/_config/pythonic_config/config.py

@@ -403,6 +403,7 @@ def infer_schema_from_config_class(
     model_cls: type["Config"],
     description: Optional[str] = None,
     fields_to_omit: Optional[set[str]] = None,
+    default: Optional[Any] = None,
 ) -> DagsterField:
     from dagster._config.pythonic_config.config import Config
     from dagster._config.pythonic_config.resource import (
@@ -433,9 +434,11 @@ def infer_schema_from_config_class(
                     " definition. 'dagster.Field' should only be used in legacy Dagster config"
                     " schemas. Did you mean to use 'pydantic.Field' instead?"
                 )
-
+            field_default = default.get(resolved_field_name) if isinstance(default, dict) else None
             try:
-                fields[resolved_field_name] = _convert_pydantic_field(pydantic_field_info)
+                fields[resolved_field_name] = _convert_pydantic_field(
+                    pydantic_field_info, default=field_default
+                )
             except DagsterInvalidConfigDefinitionError as e:
                 raise DagsterInvalidPythonicConfigDefinitionError(
                     config_class=model_cls,

python_modules/dagster/dagster/_config/pythonic_config/conversion_utils.py

@@ -84,7 +84,9 @@ def _curry_config_schema(schema_field: Field, data: Any) -> DefinitionConfigSche
 
 
 def _convert_pydantic_field(
-    pydantic_field: ModelFieldCompat, model_cls: Optional[type] = None
+    pydantic_field: ModelFieldCompat,
+    model_cls: Optional[type] = None,
+    default: Optional[Mapping[str, Any]] = None,
 ) -> Field:
     """Transforms a Pydantic field into a corresponding Dagster config field.
 
@@ -104,9 +106,11 @@ def _convert_pydantic_field(
 
     field_type = pydantic_field.annotation
     if safe_is_subclass(field_type, Config):
+        default = None
+        if pydantic_field.default and isinstance(pydantic_field.default, Config):
+            default = pydantic_field.default._get_non_default_public_field_values()  # noqa: SLF001
         inferred_field = infer_schema_from_config_class(
-            field_type,
-            description=pydantic_field.description,
+            field_type, description=pydantic_field.description, default=default
         )
         return inferred_field
     else:
@@ -116,9 +120,13 @@ def _convert_pydantic_field(
         config_type = _config_type_for_type_on_pydantic_field(field_type)
 
         default_to_pass = (
-            pydantic_field.default
-            if pydantic_field.default is not PydanticUndefined
-            else FIELD_NO_DEFAULT_PROVIDED
+            default
+            if default
+            else (
+                pydantic_field.default
+                if pydantic_field.default is not PydanticUndefined
+                else FIELD_NO_DEFAULT_PROVIDED
+            )
         )
         if isinstance(default_to_pass, Enum):
             default_to_pass = default_to_pass.name

python_modules/dagster/dagster_tests/core_tests/pythonic_config_tests/test_default_values.py

@@ -159,3 +159,51 @@ def a_job():
     a_job.execute_in_process({"ops": {"a_struct_config_op": {"config": {"a_string": "foo"}}}})
 
     assert executed["yes"]
+
+
+def test_default_values_nested_override():
+    class InnermostConfig(Config):
+        a_float: float = 1.0
+        another_float: float = 2.0
+
+    class ANestedOpConfig(Config):
+        an_int: int = 1
+        a_bool: bool = True
+        inner_config: InnermostConfig = InnermostConfig(another_float=1.0)
+
+    class AnOpConfig(Config):
+        a_string: str = "foo"
+        a_nested: ANestedOpConfig = ANestedOpConfig(an_int=5)
+
+    executed = {}
+
+    @op
+    def a_struct_config_op(config: AnOpConfig):
+        assert config.a_string == "foo"
+        assert config.a_nested.an_int == 5
+        assert config.a_nested.a_bool is True
+        assert config.a_nested.inner_config.a_float == 3.0
+        assert config.a_nested.inner_config.another_float == 1.0
+        executed["yes"] = True
+
+    from dagster._core.definitions.decorators.op_decorator import DecoratedOpFunction
+
+    assert DecoratedOpFunction(a_struct_config_op).has_config_arg()
+
+    @job
+    def a_job():
+        a_struct_config_op()
+
+    assert a_job
+
+    a_job.execute_in_process(
+        {
+            "ops": {
+                "a_struct_config_op": {
+                    "config": {"a_string": "foo", "a_nested": {"inner_config": {"a_float": 3.0}}}
+                }
+            }
+        }
+    )
+
+    assert executed["yes"]