Implement lifecycle interfaces for Core services

[google-labs-jules]

Introduced Startable and Stoppable interfaces to allow services to hook into startup and shutdown events. Updated Core to automatically register and invoke these hooks in correct order (startup: FIFO, shutdown: LIFO). Maintained 100% test coverage with new comprehensive tests.

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PR created automatically by Jules for task 10380191667075253401 started by @Snider

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Snider]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces Startable and Stoppable interfaces for managing service lifecycles, a valuable addition that enhances modularity and control over service initialization and cleanup. The implementation correctly follows a FIFO order for startup and LIFO for shutdown, as described. The use of errors.Join for aggregating errors during these lifecycle events is a modern and robust approach. The new features are backed by a comprehensive set of new tests, ensuring correctness and maintaining test coverage. I have one minor suggestion in the test code to improve conciseness by leveraging Go's method promotion with embedded structs. Overall, this is a well-executed and beneficial change.