fix: retry rate-limited samples, streak backoff, and upload timing

[code-inflation]

Summary

• add retry handling for retryable HTTP statuses and timeout transport errors in download/upload loops
• base retry backoff on consecutive retry streaks so delays scale with repeated retryable failures
• exclude failed and non-success responses from throughput stats; enforce a max-attempt budget
• print retry details only when an error occurs; keep success lines clean
• show attempts/success/skipped counters only in verbose summary output
• add deterministic mock-server tests for retry and skip behavior
• fix upload throughput timing to stop at response-header arrival (before response body drain)
• parse Retry-After before drain so retry metadata remains independent of response-body timing
• add regression tests proving delayed response bodies no longer skew upload sample duration on success or retryable failures
• update dependencies and fix clippy pipeline compatibility