Refactor: Pass shutdown receiver by value instead of mutable reference

[coderabbitai]

Description

The current implementation passes the shutdown receiver as a mutable reference (&mut shutdown_rx) into async move closures for worker tasks, creating unnecessary lifetime complexity and confusion about ownership.

Affected Code

File: src/server.rs
Lines: 289-299 and 321-333

Current Implementation Issue

Each worker owns its own broadcast::Receiver, but capturing &mut shutdown_rx inside an async move closure creates an unnecessary lifetime dance and misleads readers into thinking the receiver outlives the task.

Suggested Solution

Pass shutdown_rx by value into the closure instead of by mutable reference, so each worker owns its receiver directly.

Changes needed:

1. Update worker_task function signature to take mut shutdown_rx: broadcast::Receiver<()> instead of shutdown_rx: &mut broadcast::Receiver<()>
2. Update spawn calls to pass receiver by value instead of mutable reference
3. Remove async move complexity around receiver ownership

Benefits

• Simplifies ownership model
• Removes unnecessary lifetime complexity
• Avoids hidden self-references inside spawned futures
• Makes code more readable and maintainable

References

• PR: Implement preamble callback tests #28
• Comment: Implement preamble callback tests #28 (comment)

Requested by: leynos