test: add round-trip and wrapper tests for convert/unconvert layer

Cover all forward conversion functions (convertProjectVersion, convertFolderInfoSlice,
convertNavDoc, convertJsonStructure, convertImageCollection, etc.) and reverse
unconvert functions (unconvertNavProfileSpec, unconvertNavMenuItemSpec,
unconvertEntityMemberAccessSlice, unconvertEntityAccessRevocation,
unconvertJsonStructure, unconvertImageCollection) with fully-populated struct
inputs, nil/error passthrough, and recursive structure verification.

Includes Slice/Ptr wrapper tests for all wrapper variants with error
passthrough and nil handling. 33 tests total in convert_roundtrip_test.go.

Author: retran