test(actor): add integration coverage for core actor behaviors

[tiannian]

Motivation

• Improve test coverage for the actor crate by exercising core mailbox/message/topic flows and the preferred message path.
• Verify ServiceAddress::into_address forwarding and topic subscribe/publish semantics to prevent regressions in message routing.

Description

• Add actor/tests/actor_tests.rs which defines a TestActor and message/topic fixtures used across tests.
• Implement handlers for Add, Get, PreferredAdd, PreferredUsed, and PublishNumber, plus a NumberTopic with TopicEndpoint usage.
• Add tests covering call + send ordering, the IS_PERFERRED preferred handler path (and readback via PreferredUsed), into_address forwarding from Address<M> to the service mailbox, and one-shot topic subscribe/publish delivery.

Testing

• Ran cargo test -p serviceless, and the new integration tests all passed (4 integration tests) and crate doc-tests compiled and passed (13 doc-tests); overall test suite succeeded.

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Codex Task