[Task] Decide whether `Yaku.name` should be required

[Apricot-S]

Summary

The Yaku class currently defines name as Optional[str].
The only place where name is directly referenced in the library is the Yaku.__str__() method, which returns self.name and therefore fails when name is None.

This issue discusses whether name should be made a required field.

Motivation

• Yaku.__str__() assumes name is always a string
• Making name required would prevent unexpected None errors
• All built‑in Yaku subclasses already set name inside set_attributes()

Discussion points

• Should name be required at the type level (str instead of Optional[str])?
• If not required, should Yaku.__str__() provide a safe fallback (e.g., empty string or class name)?
• Are there use cases for a nameless Yaku?

Breaking changes

If name becomes required, the following cases will break:

• Instantiating a Yaku subclass without defining name
• Assigning name = None after instantiation