Harden retry timing option validation

[kamui]

Summary

• Validate retry timing/count options before use
• Reject invalid generated backoff values and negative custom intervals
• Preserve documented custom intervals precedence over generated backoff options

Test Plan

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

Notes

• Lazy generated interval construction was deferred after review because it can change global RNG behavior for callers that use rand inside retried blocks.
• Full bundle exec rake still hits existing repo-wide RuboCop offenses outside this PR.