remove panic paths from library runtime code

[maxcountryman]

Summary

There are several runtime/library code paths that can panic (assert!, expect, and implicit panics), which is risky for production background workers.

Evidence

• ZonedSchedule::new uses assert! on IANA name: src/scheduler.rs:63
• Worker interval conversion uses expect: src/worker.rs:835, src/worker.rs:839
• enqueue_many_with_chunk_size can panic when chunk_size == 0 (via chunks(0)): src/queue.rs:679

Why this matters

Unexpected panics can crash long-running workers/schedulers and cause avoidable downtime.

Proposed direction

• Replace panic paths with typed errors.
• Validate user-provided parameters (chunk_size > 0) early.
• Keep library behavior failure-typed rather than aborting tasks.

Acceptance criteria

• Above paths return structured errors and are covered with regression tests.