fix: action="append" on list-typed fields no longer wraps each value in an extra list

[Copilot]

When a list-annotated field uses action="append", simple_parsing was unconditionally setting nargs="*", causing each --flag value occurrence to be collected into a one-item list before being appended — silently producing [["Auth"], ["Accept"]] instead of ["Auth", "Accept"].

Changes

• simple_parsing/wrappers/field_wrapper.py — In the is_list branch of get_arg_options(), skip the nargs="*" override when action == "append". With nargs unset, argparse correctly treats each occurrence as a single value and appends it directly.
• test/test_custom_args.py — Added test_append_action_on_list_field as a regression test, and test_extend_action_on_list_field to verify action="extend" (which correctly retains nargs="*") continues to work.

@dataclasses.dataclass
class Args:
    header: list[str] = simple_parsing.field(default_factory=list, action="append")

simple_parsing.parse(Args, args=["--header", "Auth", "--header", "Accept"])
# Before: Args(header=[['Auth'], ['Accept']])
# After:  Args(header=['Auth', 'Accept'])

• Fixes action="append" on a list-typed field wraps each appended value in an extra list #366