Add Targeted Tests for Lease Recovery Timing Edge Cases

[chendbox]

Summary

Add more focused test coverage around lease recovery timing and reassignment edge cases.

Why This Matters

Lease recovery is one of the most important MLIS behaviors. Stronger edge-case coverage would make the project more trustworthy as a reference implementation.

Scope

Potential areas:

• reassignment after worker loss
• reclaim timing windows
• repeated worker failure during recovery
• visibility of reassigned work in API responses
• assignment state transitions during reclaim

Acceptance Criteria

• add targeted tests with clear scenario names
• cover at least one timing-sensitive recovery edge case
• keep the tests readable enough for contributors using the repo as a learning artifact

Good Starting Points

• tests/unit/
• tests/integration/
• src/app/control_plane/scheduler/
• src/app/data_plane/worker/