Fix sleep-disabled max elapsed retries

[kamui]

Summary

• Treat skipped sleep intervals as zero when enforcing max_elapsed_time
• Add a regression spec with stubbed monotonic time for deterministic coverage

Test Plan

• bundle exec rspec
• bundle exec rubocop lib/retriable.rb spec/retriable_spec.rb

Notes

• Full bundle exec rake still hits existing repo-wide RuboCop offenses outside this PR.