`StreamingOptions._from_argparse` silently drops env vars

[Matt711]

Part of #23740.

Describe the bug
_from_argparse passes every field explicitly, including UNSPECIFIED, which skips a
dataclass field's default_factory. default_factory only runs when the argument is
omitted, not when it's passed explicitly, so env var lookup never ran for any field set
through the CLI.

Steps/Code to reproduce bug

import argparse
import os

from cudf_polars.engine.options import StreamingOptions

os.environ["CUDF_POLARS__EXECUTOR__NUM_PY_EXECUTORS"] = "16"

parser = argparse.ArgumentParser()
StreamingOptions._add_cli_args(parser)
args = parser.parse_args([])  # nothing set on the CLI
opts = StreamingOptions._from_argparse(args)
print(opts.num_py_executors)  # UNSPECIFIED, should be 16

Expected behavior
opts.num_py_executors == 16, picked up from the env var.