Introduced `ReadEnvelopeReceiverSecretQuery` to fetch sensitive
fields (e.g., access code, phone number) for envelope receivers.
Added extension methods for dispatching the query via `IMediator`
and implemented `ReadEnvelopeReceiverSecretQueryHandler` to
process the query using repositories and AutoMapper.
Updated `EnvelopeController` with a new HTTP GET endpoint
`EnvelopeReceiverWithSecretByMediatr` to expose the query
functionality. This endpoint returns sensitive data as
`EnvelopeReceiverSecretDto` or a 404 response if no match is found.
These changes improve modularity, testability, and separation of
concerns by leveraging MediatR and CQRS patterns.
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