Summary

• Prevent retry backoff from overflowing time.Duration for large retry counts.
• Replace the floating-point exponential calculation with bounded duration doubling.
• Add a regression test covering large retry counts so jitter never receives an invalid bound.

Fixes #489

Test plan

• go test ./internal/requestconfig -run 'TestRetryDelay' -count=1
• go test ./internal/requestconfig -count=1