test(nuget): add NuGet smoke-test console app (DocumentService.Tests.NuGet)

Standalone console application that references the published DocumentService.Client
NuGet package (v1.2.2) and verifies the full happy-path against a live API instance.

- Targets net8.0; references DocumentService.Client 1.2.2 via NuGet (not project ref)
- Embeds valid.pdf as an EmbeddedResource so no file-system path is required
- Calls Client.Validation.ValidateAsync(Stream) and prints PageCount, PdfVersion,
  IsEncrypted, AttachmentCount, FileSizeBytes to stdout
- Handles HttpRequestException and InvalidOperationException with descriptive output
- Intended to run once after a new package is pushed to confirm the .nupkg bundles
  Application and Domain DLLs correctly (validates the embedding targets added in
  the previous commit)

DocumentService.Tests.NuGet/DocumentService.Tests.NuGet.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Include="valid.pdf" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="DocumentService.Client" Version="1.2.2" />
+  </ItemGroup>
+
+</Project>