fix(_models): guard against bare dict in construct_type()

[devteamaegis]

What's broken

Calling construct_type() with type_=dict (a bare, unparameterised dict) raises ValueError: not enough values to unpack (expected 2, got 0). This hits whenever a model field is annotated as plain dict rather than Dict[str, SomeType].

File "src/anthropic/_models.py", line 650, in construct_type
    _, items_type = get_args(type_)  # Dict[_, items_type]
ValueError: not enough values to unpack (expected 2, got 0)

Why it happens

The origin == dict branch unconditionally unpacks get_args(type_) as exactly two values. For a bare dict with no type parameters, get_args returns an empty tuple (), making the two-value unpack fail.

Fix

Capture get_args(type_) into a local args variable and fall back to object as items_type when args is empty. When items_type is object, every value passes through construct_type unchanged, which is the correct behaviour for an untyped dict.

Test

Added test_construct_type_bare_dict in tests/test_models.py — passes type_=dict and asserts the original dict is returned without raising.

Fixes #1619