Await endpoint.session() in null pyworker client

endpoint.session() forwards to start_endpoint_session, which is async
def — so the call returns a coroutine, not a Session, despite the
SDK's return-type annotation. Use 'async with await endpoint.session(...)'
so the coroutine resolves to a Session before entering the context.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

workers/null/client.py

@@ -40,7 +40,7 @@ async def reserve(
     lifetime = hold_for + 60
     start = time.monotonic()
     log.info("[%s] creating session (lifetime=%.0fs, hold=%.0fs)", label, lifetime, hold_for)
-    async with endpoint.session(cost=SESSION_COST, lifetime=lifetime) as s:
+    async with await endpoint.session(cost=SESSION_COST, lifetime=lifetime) as s:
         log.info("[%s] session %s open", label, s.session_id)
         try:
             await asyncio.sleep(hold_for)