This commit introduces a new Mailbox interface that abstracts the
message queue implementation for actors. Previously, actors used a
direct channel for their mailbox, which limited flexibility and made
it difficult to implement alternative mailbox strategies.
The new Mailbox interface provides methods for sending, receiving, and
draining messages, with full context support for cancellation. The
Receive method leverages Go 1.23's iter.Seq pattern, providing a clean
iterator-based API that allows natural for-range loops over messages.
The ChannelMailbox implementation maintains the existing channel-based
behavior while conforming to the new interface. It stores the actor's
context internally, ensuring both caller and actor contexts are
properly respected during send and receive operations. This simplifies
context handling compared to complex context merging approaches.
This abstraction enables future implementations such as priority
mailboxes, persistent mailboxes, or bounded mailboxes with overflow
strategies, without requiring changes to the actor implementation.
