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

Describe the bug
simple_parsing silently produces a nested (corrupted) result when a list-annotated dataclass field uses action="append", instead of raising or producing a flat list.

To Reproduce

import dataclasses
import simple_parsing

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

print(simple_parsing.parse(Args, args=["--header", "Auth", "--header", "Accept"]))

Expected behavior

Args(header=['Auth', 'Accept'])

Actual behavior

Args(header=[['Auth'], ['Accept']])

Desktop (please complete the following information):

• Version: 0.1.9
• Python version: 3.14.6

Additional context

Root cause: In simple_parsing/wrappers/field_wrapper.py, the is_list branch that builds add_argument kwargs unconditionally sets nargs="*", regardless of the requested action.

That's correct for the default store action (nargs="*" collects all tokens from one occurrence into the list), but wrong for action="append": each occurrence should be parsed as a single element, and AppendAction appends that element to a running list.

Because nargs="*" is still forced, each single --header Auth occurrence gets bundled into its own one-item list (["Auth"]) before AppendAction appends it, yielding a list of one-item lists.

No error is raised, so this fails silently.

Confirmed workaround: explicitly passing nargs=None overrides the bad default and produces the correct flat list:

header: list[str] = simple_parsing.field(default_factory=list, action="append", nargs=None)

This suggests the fix is for the is_list branch to skip (or set differently) nargs="*" when action is "append" (and possibly "extend").