Centralized `HttpRequestException` creation with a new
`CreateHttpException` method to improve code reuse and
consistency. Updated methods to handle `HttpStatusCode.NotFound`
gracefully, returning `null` where appropriate instead of
throwing exceptions. Introduced `TryGetSenderDocumentBytesByEnvelopeIdAsync`
to encapsulate fallback logic for fetching document bytes.
Removed redundant inline exception handling and standardized
error handling across methods.
TekH
6d0b78072e