Duplicate key in Enum when using both lowercase and uppercase strings

Describe the bug

enums are case-sensitive in OpenAPI spec[1] but the openapi generator does not treat them that way. When I try to use both lowercase and uppercase values in enum like so:

components:
  schemas:
    DocumentType:
      type: string
      enum: [txt, TXT]

I get this error:

Traceback (most recent call last):

  File "/Users/xxx/.local/bin/openapi-python-client", line 8, in <module>
    sys.exit(app())
             ^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/cli.py", line 175, in update
    errors = update_existing_client(
             ^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/__init__.py", line 336, in update_existing_client
    project = _get_project_for_url_or_path(
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/__init__.py", line 295, in _get_project_for_url_or_path
    openapi = GeneratorData.from_dict(data_dict, config=config)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/openapi.py", line 503, in from_dict
    schemas = build_schemas(components=openapi.components.schemas, schemas=schemas, config=config)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/__init__.py", line 386, in build_schemas
    schemas = _create_schemas(components=components, schemas=schemas, config=config)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/__init__.py", line 307, in _create_schemas
    schemas_or_err = update_schemas_with_data(ref_path=ref_path, data=data, schemas=schemas, config=config)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/schemas.py", line 114, in update_schemas_with_data
    prop, schemas = property_from_data(
                    ^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/__init__.py", line 178, in property_from_data
    return EnumProperty.build(
           ^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/enum_property.py", line 124, in build
    values = EnumProperty.values_from_list(value_list)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/Users/xxx/.local/pipx/venvs/openapi-python-client/lib/python3.12/site-packages/openapi_python_client/parser/properties/enum_property.py", line 203, in values_from_list
    raise ValueError(f"Duplicate key {key} in Enum")

ValueError: Duplicate key TXT in Enum

OpenAPI Spec File

see above

Desktop (please complete the following information):

• OS: macOS 15.x
• Python Version: 3.12
• openapi-python-client version 0.19.1

Start with openapi_python_client/parser/properties/enum_property.py, especially values_from_list, using the provided schema with enum values txt and TXT to reproduce the error. The work is done when these case-distinct enum values are accepted without a duplicate-key error and the behavior is covered by an appropriate test.