Export remaining Literal type aliases

[joaopedroassad]

Summary

• Export AssistantMessageError — the Literal union used on AssistantMessage.error.
• Export PermissionBehavior — the Literal union used on PermissionUpdate.behavior.
• Export PermissionUpdateDestination — the Literal union used on PermissionUpdate.destination.
• Export ThinkingDisplay — the Literal union used on ThinkingConfigAdaptive.display and ThinkingConfigEnabled.display.

All four aliases are already used as field types on publicly-exported classes, but downstream SDK wrappers that want to type-annotate variables with these unions currently have to import from the private claude_agent_sdk.types module. Same shape as #951 (EffortLevel).

Testing

• .venv/bin/python -m pytest tests/test_types.py -q
• .venv/bin/python -m ruff check src/claude_agent_sdk/__init__.py tests/test_types.py
• .venv/bin/python -m ruff format --check src/claude_agent_sdk/__init__.py tests/test_types.py
• git diff --check