Add backoff for failed dequeue operations

[samuel-williams-shopify]

Summary

• retry failed dequeue operations using configurable exponential backoff with jitter
• cap retry delays and reset them after a successful dequeue
• treat an empty result from the indefinitely blocking Redis dequeue as an I/O error
• cover retry behavior, delay reset, and empty dequeue results

Verification

• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• 100% line coverage from a fresh coverage database

Fixes #6