The `GetDocumentOfReceiver` method was introduced to handle
retrieving a document for a specified envelope key. It is
secured with the `[Authorize(Policy = AuthPolicy.Receiver)]`
attribute and responds to HTTP GET requests with a route
parameter `envelopeKey`.
The previous logic for extracting and validating the
`envelopeId` from user claims was removed. This was replaced
with a call to `User.GetEnvelopeIdOfReceiver()` for improved
clarity and maintainability.
The method uses the `mediator` to send a `ReadDocumentQuery`
and returns a `NotFound` response if the document's `ByteData`
is empty.
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