Fix required fields inconsistently rendering defaults (#3048)

__set_validate_default_on_fields set validate_default=True on fields
with model references without checking field.required, causing required
model-ref fields to render defaults while required scalar fields didn't.

Skip required fields (unless use_default_with_required) so all required
fields consistently have no defaults rendered.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: butvinm

src/datamodel_code_generator/parser/base.py

@@ -2177,6 +2177,8 @@ def __set_validate_default_on_fields(  # noqa: PLR6301
             if isinstance(model, Enum):
                 continue
             for model_field in model.fields:
+                if model_field.required and not model_field.use_default_with_required:
+                    continue
                 if model_field.default is None or model_field.default is UNDEFINED:
                     continue
                 if isinstance(model_field.default, Member):

tests/data/expected/main/graphql/pydantic_v2_empty_list_default.py

@@ -26,8 +26,8 @@ class Container(BaseModel):
 
 
 class PodSpec(BaseModel):
-    container_list: list[Container] = Field([], validate_default=True)
-    container_list_or_none: list[Container | None] = Field([], validate_default=True)
+    container_list: list[Container]
+    container_list_or_none: list[Container | None]
     container_or_none_list_or_none: list[Container | None] | None = Field(
         [], validate_default=True
     )