2.1.21: Fix custom_properties diffing when config uses `property_name` (#978)

* Initial plan

* Handle alternate custom property name shapes in normalize

* Add explicit object guard in custom property normalization

* Support property_name in custom_properties config entries

* Polish custom properties test descriptions

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>