feat: Add websocket max heap guardrail

[aloukissas]

Summary

• Add an optional :max_heap_size socket option that caps each websocket process heap.
• Convert byte limits to BEAM words and apply Process.flag(:max_heap_size, %{kill: true, error_logger: true}) during socket init.
• Emit telemetry when the heap guardrail is applied, document the option, and add integration coverage.

Testing

• mix test