Fix duration overflow crashes and bound retry attempts

[ryanfowler]

Summary

• Reject oversized duration values before converting to Duration, using the same maximum bound across CLI and config parsing.
• Validate --retry with checked addition so the total attempt count cannot overflow.
• Add unit coverage for oversized durations and retry overflow handling.

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features