Set null pyworker client cost to 1

Match the server-side workload_calculator (1.0) so the autoscaler routing
hint is consistent with what the worker reports. A null reservation is a
unitless slot — no reason for client cost to be 100.

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

workers/null/client.py

@@ -36,7 +36,7 @@ async def reserve(
     start = time.monotonic()
     log.info("[%s] POST /reserve duration=%ss", label, duration)
     try:
-        resp = await endpoint.request("/reserve", payload, cost=100)
+        resp = await endpoint.request("/reserve", payload, cost=1)
         elapsed = time.monotonic() - start
         log.info("[%s] returned after %.1fs: %s", label, elapsed, resp.get("response"))
         return resp["response"]