fix(parse): reject unrepresentable dynamic object schemas

[uuzzrm]

Summary

Fixes #1817.

When an object schema uses patternProperties, propertyNames, or unevaluatedProperties without explicit properties, the structured-output schema transformer previously discarded the dynamic-key rule and emitted an object with no properties and additionalProperties: false. That silently made every non-empty value invalid.

This change raises a clear ValueError for those unrepresentable shapes instead of sending an unsatisfiable schema. Objects that already have explicit properties keep their existing behavior; unsupported dynamic-key constraints remain in the description as before.

Tests

• UV_PYTHON='>=3.9,<3.10' ./scripts/test tests/lib/_parse/test_transform.py
  • Pydantic v2: 18 passed
  • Pydantic v1: 18 passed
  • MCP v2: skipped because MCP 2 requires Python 3.10+
• UV_PYTHON='>=3.10,<3.11' ./scripts/test tests/lib/_parse/test_transform.py
  • Pydantic v2: 18 passed
  • Pydantic v1: 18 passed
  • MCP v2: 39 passed
• uv run pyright src/anthropic/lib/_parse/_transform.py: 0 errors
• uv run ruff check src/anthropic/lib/_parse/_transform.py tests/lib/_parse/test_transform.py: passed
• uv run ruff format --check src/anthropic/lib/_parse/_transform.py tests/lib/_parse/test_transform.py: passed