fix: stop sync client busy loop after disconnect

[gitwillsky]

Summary

• stop the synchronous callback worker when Engine.IO reaches a terminal state
• only reconnect incomplete Engine.IO responses when reconnect is enabled
• cover stopped sockets and transport errors with a focused unit test

Root cause

The synchronous client consumes its internal iterator on a background thread. When Engine.IO
stops, the iterator currently converts Ok(None) into another error item. With automatic
reconnect disabled, the callback loop ignores that error and polls the already-closed transport
again immediately. Each failed connection can therefore leave one busy-looping thread behind.

This change makes a stopped Engine.IO iterator terminal and explicitly stops the callback loop on
poll errors or namespace disconnects when reconnect is disabled. Normal packets do not acquire the
builder lock.

Validation

• cargo check -p rust_socketio
• cargo fmt --all -- --check
• consumer reproduction before the fix: two failed connections produced two hot worker threads
  and 148-199% CPU
• consumer reproduction after the fix: ten forced namespace disconnects remained at 0.0% CPU with
  a stable sleeping thread count