feat: Simplify service event listening API

[vobradovich]

Summary

• Replace the Service::listener().listen().await flow with direct Service::listen().await
• Remove the intermediate ServiceListener type
• Update gtest and gclient listener return types with explicit use<...> captures
• Update demo, partial IDL, and client-gen tests to use the new direct listener API

[gemini-code-assist]

Code Review

This pull request simplifies the event listening API by removing the ServiceListener struct and allowing users to call .listen() directly on service clients. It also introduces the new Rust precise capturing syntax (use<...>) for the Listener trait. Feedback was provided regarding inconsistencies in the capture lists within the GclientEnv and GtestEnv implementations, which should include Self to match the trait definition.