types/docs: add compact message reprs and clarify plugin MCP permissions

[however-yir]

Summary

• add compact __repr__ implementations for message/content block dataclasses so streamed messages are readable without custom helper formatting
• add a README section clarifying plugin MCP permission behavior:
  • plugin MCP tools still go through the standard permission pipeline
  • they do not bypass permission_mode, can_use_tool, or hooks
  • allowed_tools controls auto-approval
• add test coverage for compact message repr behavior

Issues addressed

• Fixes Support reprs for message types #78
• Fixes Unclear Documentation: Do Plugin MCP Servers Automatically Receive Permissions? #448

Testing

• python -m pytest tests/test_types.py

[however-yir]

Hi maintainers, friendly ping for review.

This PR addresses #78 and #448 by:

• adding compact, readable __repr__ implementations for message/content dataclasses
• clarifying plugin MCP permission behavior in README (standard permission pipeline, no bypass)
• adding/covering repr behavior in tests/test_types.py

Validation run:

• python -m pytest tests/test_types.py (passes locally)

Happy to trim or reshape the repr output if you prefer a different style.