Refactor: move where we call field.coerce_result.

[myronmarston]

Previously, we called it from GarphQLAdapter#call, applying it to all resolved fields. However, we only need to use it in the aggregation GroupedBy resolver, where we get DayOfWeek enum values that we may need to coerce. There are no other cases where it is needed.

This allows us to simplify GarphQLAdapter#call so that it now only does two things:

• Identifies the resolver to dispatch to.
• Dispatches to that resolver.

This will allow us to further optimize by leveraging built-in functionality of the GraphQL gem to have it dispatch to the appropriate resolver in a more performant manner.

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