Add envelope validation to BurnAnnotsToPDF function

Retrieve envelope by ID before burning annotations. Throw an exception if not found, and return the original PDF if the envelope is read-only. This adds validation and early exit logic before processing signatures and annotations.

EnvelopeGenerator.CommonServices/Jobs/FinalizeDocument/PDFBurner.vb

@@ -37,6 +37,15 @@ Namespace Jobs.FinalizeDocument
         Public Function BurnAnnotsToPDF(pSourceBuffer As Byte(), pInstantJSONList As List(Of String), envelopeId As Integer) As Byte()
             'read the elements of envelope with their annotations
             Using scope = Factory.Shared.ScopeFactory.CreateScope()
+
+                Dim envRepo = scope.ServiceProvider.Repository(Of Envelope)()
+                Dim envelope = envRepo.Where(Function(env) env.Id = envelopeId).FirstOrDefault()
+                If envelope Is Nothing Then
+                    Throw New BurnAnnotationException($"Envelope with Id {envelopeId} not found.")
+                ElseIf envelope.ReadOnly Then
+                    Return pSourceBuffer
+                End If
+
                 Dim sigRepo = scope.ServiceProvider.Repository(Of Signature)()
                 Dim elements = sigRepo _
                     .Where(Function(sig) sig.Document.EnvelopeId = envelopeId) _