Remove dead SafeDivide overload and unreachable top-product branch #606

Summary

Remove two branches in EstateReportingAPI.BusinessLogic/ReportingManager.cs that are not reachable through the current production code paths:

1. Integer SafeDivide overload (around lines 555–560)

   • The integer overload has no callers.
   • The reporting manager only uses the decimal overload.

2. Null top-product branch (around line 2127)

   • BuildTopProductMetric is only called after grouped transactions have been confirmed non-empty.
   • FirstOrDefault() therefore cannot return null for the supplied list.
   • Remove the redundant null branch, or simplify the method so the control flow reflects the invariant.

Acceptance criteria

• Remove the unused integer SafeDivide overload.
• Remove or simplify the unreachable null top-product branch.
• Preserve existing report behaviour.
• Run the business-logic unit tests and API tests.
• Confirm coverage and build remain clean.