Expand unit coverage for report calculations and edge cases #597

Problem

The repository has broad integration coverage, but business-logic unit coverage is comparatively limited. Important calculations and edge conditions are concentrated in the large reporting manager.

Relevant areas:

• EstateReportingAPI.BusinessLogic.UnitTests
• EstateReportingAPI.Tests
• EstateReportingAPI.IntegrationTests
• EstateReportingAPI.BusinessLogic/ReportingManager.cs

Scope

Add focused tests for:

• Empty datasets.
• Date boundaries and invalid ranges.
• Failed/declined transaction totals.
• Settlement and fee totals.
• Merchant/operator aggregation.
• Duplicate or missing balance entries.
• Pagination and maximum query limits.
• Tenant isolation behavior.

Acceptance criteria

• Core report calculations have direct unit tests.
• Edge-case tests assert both result values and failure/not-found behavior.
• Integration tests remain focused on HTTP, authentication, serialization, and database wiring.
• The test suite documents the expected behavior for zero-result reports.