Consider the zone radius in proximity distance calculation (#138819)

* Fix proximity distance calculation

The distance is now calculated to the edge of the zone instead of the centre

* Adjust proximity test expectations to corrected distance calculation

* Add proximity tests for zone changes

* Improve comment on proximity distance calculation

Co-authored-by: Michael <35783820+mib1185@users.noreply.github.com>

* Apply suggestions from code review

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Co-authored-by: Michael <35783820+mib1185@users.noreply.github.com>