fix: handle bare container types

[ynachiket]

Fixes #1619.
Fixes #1626.
Fixes #1628.

Summary

• treat bare dict and list annotations as untyped containers in construct_type instead of indexing missing type args
• treat bare dict annotations as untyped in sync and async transform paths
• add focused regression coverage for the reported bare-container crashes

Testing

• PYTHONPATH=src /tmp/anthropic-pdlc042-venv/bin/python -m pytest tests/test_models.py::test_construct_type_bare_container_types tests/test_transform.py::test_bare_dict_type -q (3 passed)
• PYTHONPATH=src /tmp/anthropic-pdlc042-venv/bin/python -m pytest tests/test_models.py tests/test_transform.py -q (117 passed)
• PYTHONPATH=src /tmp/anthropic-pdlc042-venv/bin/python -m py_compile src/anthropic/_models.py src/anthropic/_utils/_transform.py tests/test_models.py tests/test_transform.py
• git diff --check