test: mutation testing for scheduler module

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing for the scheduler — handles periodic tasks like order expiration, dev fee processing, and stale order cleanup.

Scope

• src/scheduler.rs — All scheduled tasks

Goal

• Run cargo mutants --file src/scheduler.rs
• Kill surviving mutants
• Target: >70% mutation score

Priority Mutants to Kill

• Order expiration timing logic
• Job scheduling intervals
• Stale order detection and cleanup
• Dev fee payment retry scheduling
• Rate limiter expiration

Acceptance Criteria

• Baseline mutation report
• Critical mutants in expiration logic killed
• Critical mutants in cleanup logic killed
• Mutation score documented in PR

Ref: #618