fix(routing): 额度饱和时解除任务候选固定

已取得并发租约的任务继续保持候选绑定；仅当已绑定平台的 RPM 或 TPM 已满且存在可用候选时，允许任务在执行前原子迁移到下一平台，避免等待窗口重置。\n\n验证：\n- go test ./internal/runner -count=1\n- go vet ./internal/runner

apps/api/internal/runner/admission.go

@@ -293,6 +293,10 @@ func pinCandidatesToTaskAdmission(
 	if pinnedIndex <= 0 {
 		return candidates, false
 	}
+	pinnedCandidate := candidates[pinnedIndex]
+	if providerRateQuotaFull(pinnedCandidate) && !providerRateQuotaFull(candidates[0]) {
+		return candidates, false
+	}
 	out := append([]store.RuntimeModelCandidate(nil), candidates...)
 	pinned := out[pinnedIndex]
 	copy(out[1:pinnedIndex+1], out[:pinnedIndex])
@@ -300,6 +304,12 @@ func pinCandidatesToTaskAdmission(
 	return out, true
 }
 
+func providerRateQuotaFull(candidate store.RuntimeModelCandidate) bool {
+	metrics := candidate.LoadMetrics
+	return (metrics.RPMLimit > 0 && metrics.RPMCurrent >= metrics.RPMLimit) ||
+		(metrics.TPMLimit > 0 && metrics.TPMCurrent >= metrics.TPMLimit)
+}
+
 func (s *Service) tryTaskAdmission(ctx context.Context, task store.GatewayTask, plan taskAdmissionPlan, waiterID string) (store.TaskAdmissionResult, error) {
 	return s.tryTaskAdmissionWithAdmittedHook(ctx, task, plan, waiterID, nil)
 }