InMemorySessionService allows duplicate events to be appended during concurrent state broadcasts #5723

Description

In InMemorySessionService.append_event, there is no deduplication check before appending an Event to session.events. When an orchestrator or background task broadcasts shared state updates to multiple concurrent agent sessions, race conditions can cause the exact same event ID to be appended multiple times to a session's history.

Proposed Solution

Add a simple, universal deduplication check at the beginning of append_event:

async def append_event(self, session: Session, event: Event) -> Event:
    if event.partial:
        return event
    if any(e.id == event.id for e in session.events):
        return event
    # ...