perf: sequential PubSub dispatch blocks all subscribers on slow client

[csmith49]

Problem

When an event fires, PubSub.__call__ iterates all subscribers sequentially and awaits each one. If one WebSocket subscriber is slow (congested client), it delays event delivery to all other subscribers for that conversation.

Location: pub_sub.py:63–74

Evidence

Code review of pub_sub.py:63–74.

Severity: Medium

Impact of fix

Slow clients don't block other subscribers.

Suggested fixes

• Use asyncio.gather for concurrent dispatch
• Or give each subscriber its own asyncio queue with independent drain

Discovered during profiling investigation (code review), May 2026. openhands-agent-server v1.19.1.