fix: use List instead of list for Python 3.9 compatibility

Replace lowercase 'list' type hints with uppercase 'List' from typing module
to support Python 3.9. The lowercase generic types require either:
- Python 3.9+ with 'from __future__ import annotations'
- Python 3.10+ without future import

Since the project supports Python 3.9, use the typing.List import instead.

Fixes ty type checker errors in CI.

Author: Doug Borg

.claude/settings.local.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(poetry run pytest:*)"
+    ],
+    "deny": [],
+    "ask": []
+  }
+}

src/openapi_python_generator/language_converters/python/model_generator.py

@@ -143,7 +143,7 @@ def _wrap_optional(type_str: str, required: bool) -> str:
     return f"Optional[{type_str}]"
 
 
-def _collect_unique_imports(conversions: list[TypeConversion]) -> Optional[List[str]]:
+def _collect_unique_imports(conversions: List[TypeConversion]) -> Optional[List[str]]:
     """
     Safely collect and deduplicate imports from conversions.
 
@@ -246,7 +246,7 @@ def _convert_array_type(
 
 def _convert_composite_schema(
     kind: str,
-    sub_schemas: list[Union[Schema, Reference]],
+    sub_schemas: List[Union[Schema, Reference]],
     required: bool,
     model_name: Optional[str],
 ) -> TypeConversion: