fix(combo): reconcile #11360 retry-loop persisted-cooldown recheck return shape

The retry-loop recheck returned a non-conforming {ok:false, reason} object
that breaks typecheck against the established {ok, response?} contract used
everywhere else in this function. Aligns with the pre-dispatch skip pattern
(return null after fallbackCount++), matching the PR's own intent: skip this
target and move to the next, not error the whole attempt.

This is a live fix — the broken shape reached origin/release/v3.8.50 via
#11360's own squash-merge and was breaking typecheck:core until now.

open-sse/services/combo.ts

@@ -1524,7 +1524,8 @@ async function handleComboChatInner({
             );
             if (persistedRetrySkip) {
               log.info("COMBO", persistedRetrySkip);
-              return { ok: false, reason: "persisted_cooldown" };
+              if (i > 0) fallbackCount++;
+              return null;
             }
           }