test: add encryptedPdf.pdf test fixture with /Encrypt in trailer

Minimal hand-crafted PDF containing a Standard encryption dictionary
referenced from the trailer (/Encrypt 4 0 R). Used by DetectEncryption
unit tests to verify byte-level encryption detection.

Registered as EmbeddedResource in DocumentService.Tests.csproj.

DocumentOperator.Tests/DocumentService.Tests.csproj

@@ -13,12 +13,14 @@
     <None Remove="TestData\Pdfs\pdfWithSwissQRCode.pdf" />
     <None Remove="TestData\Pdfs\valid.pdf" />
     <None Remove="TestData\Pdfs\pdfWithMoreThanOneAttachment.pdf" />
+    <None Remove="TestData\Pdfs\encryptedPdf.pdf" />
   </ItemGroup>
 
   <ItemGroup>
     <EmbeddedResource Include="TestData\Pdfs\pdfWithSwissQRCode.pdf" />
     <EmbeddedResource Include="TestData\Pdfs\valid.pdf" />
     <EmbeddedResource Include="TestData\Pdfs\pdfWithMoreThanOneAttachment.pdf" />
+    <EmbeddedResource Include="TestData\Pdfs\encryptedPdf.pdf" />
   </ItemGroup>
 
   <ItemGroup>

DocumentOperator.Tests/TestData/Pdfs/encryptedPdf.pdf

@@ -0,0 +1,30 @@
+%PDF-1.4
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+
+2 0 obj
+<< /Type /Pages /Kids [3 0 R] /Count 1 >>
+endobj
+
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] >>
+endobj
+
+4 0 obj
+<< /Filter /Standard /V 2 /R 3 /Length 128 /P -3904 /O (OwnerHash12345678901234567890123456) /U (UserHash12345678901234567890123456) >>
+endobj
+
+xref
+0 5
+0000000000 65535 f 
+0000000009 00000 n 
+0000000058 00000 n 
+0000000115 00000 n 
+0000000206 00000 n 
+
+trailer
+<< /Size 5 /Root 1 0 R /Encrypt 4 0 R >>
+startxref
+370
+%%EOF