refactor(PDFBurner): extract instant JSON annotation burning logic into separate method

- Moved annotation burning logic from `BurnAnnotsToPDF` into new method `BurnInstantJSONAnnotsToPDF`
- Simplified `BurnAnnotsToPDF` to handle only background generation and call the new method
- Improved code readability and separation of concerns

EnvelopeGenerator.CommonServices/Jobs/FinalizeDocument/PDFBurner.vb

@@ -49,6 +49,10 @@ Namespace Jobs.FinalizeDocument
                 pSourceBuffer = doc.Background(elements).ExportStream().ToArray()
             End Using
 
+            Return BurnInstantJSONAnnotsToPDF(pSourceBuffer, pInstantJSONList)
+        End Function
+
+        Public Function BurnInstantJSONAnnotsToPDF(pSourceBuffer As Byte(), pInstantJSONList As List(Of String)) As Byte()
             Dim oResult As GdPictureStatus
             Using oSourceStream As New MemoryStream(pSourceBuffer)
                 ' Open PDF