fix: handle bare dict annotation in construct_type()

[gavin913-lss]

What breaks

construct_type() in src/openai/_models.py crashes with ValueError when type_ is a bare, unparameterised dict (no [K, V] type arguments).

from openai._models import construct_type
result = construct_type(value={key: value}, type_=dict)
# ValueError: not enough values to unpack (expected 2, got 0)

Fixes #3341

Root cause

Line 660 does _, items_type = get_args(type_) after checking origin == dict. For bare dict, get_args(dict) returns an empty tuple (), and unpacking with two targets raises ValueError.

Fix

Add a length check before unpacking. When get_args() returns fewer than 2 elements (bare dict), return the value as-is since there are no type parameters to recurse into.

args = get_args(type_)
if len(args) < 2:
    return value  # bare dict, no type parameters
_, items_type = args

Tests

Added test_bare_dict_annotation() covering:

• Bare dict with string values
• Bare dict with int values
• Empty dict

All existing test_models.py tests continue to pass.