[Bugfix] Fix ZMQ port TOCTOU race in shm_broadcast.py

[RTCartist]

Summary

• Fix TOCTOU (time-of-check-time-of-use) race condition in MessageQueue.__init__ where get_open_port() discovers a port, but another process (e.g., Ray) can claim it
  before the ZMQ XPUB socket binds — causing zmq.error.ZMQError: Address already in use
• Replace with late binding (port=0), letting the OS assign the port atomically at bind() time, then read back the real address via zmq.LAST_ENDPOINT
• Follows the same pattern already merged in the DP coordinator (PR Fix DP coordinator ZMQ port TOCTOU #37452)

Motivation

In multi-node deployments (e.g., 2P 4D with DeepSeek-V3), engine startup takes ~5 minutes after port selection. During this window, other services can claim the
pre-selected port. This fix eliminates that race window entirely.

Changes

• vllm/distributed/device_communicators/shm_broadcast.py:
  • Removed get_open_port() call for remote subscriber socket
  • Bind to tcp://{connect_ip}:0 instead of a pre-selected port
  • Read actual bound endpoint via self.remote_socket.getsockopt(zmq.LAST_ENDPOINT).decode()
  • Removed unused get_open_port import

Existing PRs

PRs #30520, #35977, and #39496 address the same issue but have been stalled with merge conflicts for 3+ months. This PR is a minimal, focused fix for the one remaining
TOCTOU site (shm_broadcast.py), since the coordinator side was already fixed by PR #37452.

Test plan

• Existing shm_broadcast tests pass (the fix is a drop-in replacement — same bind semantics, same address format from LAST_ENDPOINT)
• Multi-node deployments no longer hit Address already in use on the remote XPUB socket

AI disclosure

This PR was developed with AI assistance (Claude). All code has been reviewed and understood by the human submitter.

Closes #28498

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