zoho: fix flaky folder list limiter test under concurrent listings

The folder list limiter granted the caller-supplied time immediately in
the burst phase, so concurrent callers observing time.Now() out of order
could record grant times that moved backwards. The sliding safety log
indexes grants as an ordered history, so out-of-order grants could also
breach the rolling-window cap. Clamp each grant to be at or after the
previous one so grant times are always monotonic.

backend/zoho/zoho.go

@@ -447,6 +447,12 @@ const folderListSafetyMargin = 500 * time.Millisecond
 func (l *folderWindowLimiter) reserve(now time.Time) time.Time {
 	l.mu.Lock()
 	defer l.mu.Unlock()
+	// Never let a grant move backwards, even if concurrent callers observe
+	// time.Now() out of order: the sliding safety log below indexes grants as
+	// an ordered history, so out-of-order grant times would corrupt the cap.
+	if now.Before(l.lastGrant) {
+		now = l.lastGrant
+	}
 	if l.windowStart.IsZero() {
 		l.windowStart = now
 	}