Fix retry body replay and jitter panic; drive delays via internal/clock

Gate the retry decision on body rewindability: an idempotent request whose
body cannot be replayed (no GetBody) is now returned as-is instead of looping
with an empty body or surfacing a stale, already-drained response. Guard
ExponentialBackoff against rand.Int64N panicking when delay/2 rounds to zero.
Use internal/clock for inter-attempt delays so retry timing is consistent with
the rest of the codebase and testable without real sleeps.

retry/backoff.go

@@ -44,8 +44,11 @@ func (b *exponentialBackoff) Delay(attempt int) time.Duration {
 	}
 
 	if b.withJitter {
-		jitter := time.Duration(rand.Int64N(int64(delay / 2)))
-		delay += jitter
+		// Guard against rand.Int64N panicking on a non-positive argument when
+		// delay is small enough that delay/2 rounds to zero.
+		if half := int64(delay / 2); half > 0 {
+			delay += time.Duration(rand.Int64N(half))
+		}
 	}
 
 	if delay > b.max {