fix(dartgen): convert mapped list fields to lists in toJson

[fondoger]

What this PR does

This fixes Dart code generated by goctl api dartgen for class list fields in toJson().

Previously, generated toJson() returned a MappedListIterable for fields such as List<Foo> / List<Foo?>:

'items': items.map((i) => i?.toJson())

That value cannot be encoded by dart:convert when calling jsonEncode(obj.toJson()).

This PR appends .toList() in both the current and legacy dart data templates:

'items': items.map((i) => i?.toJson()).toList()

Fixes #5629.

Tests

cd tools/goctl
go test ./api/dartgen
go test ./api/...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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