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.
Aleksey Shakhmatov
43d3ecfba1