Add regression coverage for createDisputeSLAHandlers failure handling

[olufunsoolutayo]

##closes #1067

## Summary

Adds regression coverage for `createDisputeSLAHandlers` report date validation. No production behavior or public contract changes.

## Coverage

- Valid dates are parsed and passed to the SLA service.
- Missing, non-string, and malformed dates return the expected `400` response.
- Equal or reversed date ranges return `400`.
- Invalid inputs do not invoke the service or forward errors to middleware.

## Validation

- Dispute route tests: 41 passed.
- Dispute route and SLA service suites: 108 passed.
- `git diff --check`: passed.
- Build and lint remain blocked by pre-existing errors outside the changed test sections.

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