test: add missing unit tests for operator rotation, threshold window,…

[hardcordev]

Closes #773
Closes #772
Closes #769
Closes #774

Summary

• Closes Add Tests for Operator Rotation Contract Module #773 — adds contracts/soroban/tests/operator_rotation.test.rs (15 tests) covering add, remove,
  reactivation, handover, last-operator protection, audit fields, and event emission
• Closes Add Tests for Threshold Window Contract Module #772 — adds contracts/soroban/tests/threshold_window.test.rs (20 tests) covering
  create/update/remove, threshold evaluation (breach, no-breach, zero reference), unit conversions
  (seconds/minutes/hours), validation panics, and MAX_WINDOWS limit
• Closes Add Tests for Export Job #769 — adds backend/tests/jobs/export.job.test.ts (11 tests) covering singleton pattern, queue
  configuration, addExportJob payload/jobId, and close()
• Closes Add Unit Tests for Email Service #774 — adds backend/tests/services/email.service.test.ts (18 tests) covering send, retry-on-failure,
  max-attempts exhaustion, unsubscribed/bounced skip, delivery stats, and SMTP connection verification

Test plan

• cargo test --test operator_rotation_test — 15/15 passed
• cargo test --test threshold_window_test — 20/20 passed
• npx vitest run tests/jobs/export.job.test.ts — 11/11 passed
• npx vitest run tests/services/email.service.test.ts — 18/18 passed

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