fix(retry): restore jitter in _calculate_backoff_delay

[sridipbasu]

Summary

Fixes a jitter calculation bug in _calculate_backoff_delay that caused
retry back-off jitter to always evaluate to zero, making retries fully
deterministic instead of randomized.

Root Cause

Before (broken)

random_wait_ms = int(random() - 0.5) * 2 * delay_ms * randomness_factor
#                ^^^^^^^^^^^^^^^^^^^
#                int() applied too early — always truncates to 0

random() - 0.5 produces values in the range [-0.5, 0.5). Applying
int() before scaling truncates every possible value to 0, causing
the jitter term to always evaluate to zero.

After (correct)

random_wait_ms = int((random() - 0.5) * 2 * delay_ms * randomness_factor)
#                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#                    int() applied after scaling

Applying int() after the full expression preserves randomized jitter
while still returning an integer millisecond delay.