test(unit): migrate unit tests to Stream API

Application Handler Tests:
- ValidatePdfHandlerTests: PdfStream = new MemoryStream(pdfBytes)
- ValidatePdfAQueryHandlerTests: PdfStream = new MemoryStream(pdfBytes)
- CheckPdfAttachmentsQueryHandlerTests: PdfStream = new MemoryStream(pdfBytes)
- Remove Base64/PdfBytes property usage

Infrastructure Tests:
- DevExpressSwissQrCodeProcessorTests: LoadTestPdf() returns Stream
- All test methods use 'using var stream' pattern
- Add test: ExtractSwissQrCodeAsync_StreamNotAtBeginning_ThrowsBadRequestException

Result: All unit tests pass with Stream-based API

DocumentOperator.Tests/Unit/Application/Features/ValidatePdf/ValidatePdfHandlerTests.cs

@@ -27,7 +27,7 @@ public class ValidatePdfHandlerTests
     {
         // Arrange
         var pdfBytes = "%PDF"u8.ToArray(); // "%PDF"
-        var query = new ValidatePdfQuery { PdfBytes = pdfBytes };
+        var query = new ValidatePdfQuery { PdfStream = new MemoryStream(pdfBytes) };
 
         var domainMetadata = new PdfMetadata(
             pageCount: 5,
@@ -74,7 +74,7 @@ public class ValidatePdfHandlerTests
     {
         // Arrange
         var pdfBytes = "%PDF"u8.ToArray(); // "%PDF"
-        var query = new ValidatePdfQuery { PdfBytes = pdfBytes };
+        var query = new ValidatePdfQuery { PdfStream = new MemoryStream(pdfBytes) };
 
         _mockPdfProcessor
             .Setup(x => x.ValidateAsync(It.IsAny<Stream>()))