refactor: extract vllm-managed-engine crate

[BugenZhao]

Extract the code logic for spawning and monitoring the managed engine, along with the CLI args definition and repartitioning to a separate crate.

[gemini-code-assist]

Code Review

This pull request refactors the managed engine logic into a new standalone crate, vllm-managed-engine, and updates the zeromq dependency to its official crates.io release. The CLI argument parsing for the serve command has been restructured to use the new crate's ManagedEngineArgs, and the previous serve_validate logic was replaced with a more generic repartitioning utility. Feedback from the review identifies a race condition in the ephemeral port allocation logic and recommends replacing .expect() calls in the library with structured error handling to prevent application panics.