test: add regression test for UnionMode runtime access

Add test that verifies UnionMode is available at runtime for Pydantic
model_rebuild(). This test runs in a subprocess to ensure a clean namespace
without pytest's import side effects, properly detecting when UnionMode
is incorrectly placed inside TYPE_CHECKING block.

Author: koxudaxi

tests/data/expected/main/generate_config_union_mode.py

@@ -0,0 +1,10 @@
+# generated by datamodel-codegen:
+#   filename:  <stdin>
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field
+
+
+class Model(BaseModel):
+    value: str | int | None = Field(None, union_mode='left_to_right')

tests/main/test_main_general.py

@@ -26,6 +26,7 @@
 from datamodel_code_generator.arguments import _dataclass_arguments
 from datamodel_code_generator.config import GenerateConfig
 from datamodel_code_generator.format import CodeFormatter, PythonVersion
+from datamodel_code_generator.model.pydantic_v2 import UnionMode
 from datamodel_code_generator.parser.openapi import OpenAPIParser
 from tests.conftest import assert_output, create_assert_file_content, freeze_time
 from tests.main.conftest import (
@@ -2062,6 +2063,21 @@ def test_generate_with_config_object(output_file: Path) -> None:
     assert "user_name" in content
 
 
+@pytest.mark.skipif(pydantic.VERSION < "2.0.0", reason="GenerateConfig requires Pydantic v2")
+def test_generate_config_with_union_mode() -> None:
+    """Test GenerateConfig with union_mode field."""
+    config = GenerateConfig(
+        output_model_type=DataModelType.PydanticV2BaseModel,
+        union_mode=UnionMode.left_to_right,
+        disable_timestamp=True,
+    )
+    result = generate(
+        input_='{"type": "object", "properties": {"value": {"anyOf": [{"type": "string"}, {"type": "integer"}]}}}',
+        config=config,
+    )
+    assert_output(result, EXPECTED_MAIN_PATH / "generate_config_union_mode.py")
+
+
 @pytest.mark.skipif(pydantic.VERSION < "2.0.0", reason="GenerateConfig requires Pydantic v2")
 def test_generate_with_config_and_kwargs_raises_error(output_file: Path) -> None:
     """Test generate() raises error when both config and kwargs are provided."""