actor: add tests for mailbox implementation

This commit adds thorough test coverage for the new Mailbox interface
and ChannelMailbox implementation. The tests verify correct behavior
across various scenarios including successful sends, context
cancellation, mailbox closure, and concurrent operations.

The test suite specifically validates that the mailbox respects both
the caller's context and the actor's context during send and receive
operations. This ensures that actors properly shut down when their
context is cancelled, and that callers can cancel operations without
affecting the actor's lifecycle.

Additional tests cover edge cases such as zero-capacity mailboxes
(which default to a capacity of 1), draining messages after closure,
and concurrent sends from multiple goroutines. The concurrent test
uses 10 senders each sending 100 messages to verify thread-safety
and proper message ordering.

All tests pass with the race detector enabled, confirming the
implementation is free from data races.

Author: Roasbeef