fix: support list-form JSON schema types

[nightcityblade]

Fixes #1764

Summary

• canonicalize list-form JSON Schema types into anyOf branches
• keep shared descriptions, titles, and enums on the union while applying sibling constraints to non-null branches
• add regression coverage for simple and constrained nullable schemas

Test plan

• uv run pytest tests/lib/_parse -n0 (24 passed)
• uv run ruff check src/anthropic/lib/_parse/_transform.py tests/lib/_parse/test_transform.py
• uv run ruff format --check src/anthropic/lib/_parse/_transform.py tests/lib/_parse/test_transform.py
• uv run pyright src/anthropic/lib/_parse/_transform.py