Add project references and fix encoding issue in tests

Added project references to establish dependencies between
the API, Application, Domain, and Infrastructure projects.
Updated `DigitalData.EmailProfiler.Tests.csproj` to include
references to all layers for testing purposes. Fixed a BOM
encoding issue in the test project file. Added xUnit usage
directive to ensure proper test framework integration.

src/DigitalData.EmailProfiler.API/DigitalData.EmailProfiler.API.csproj

@@ -16,4 +16,10 @@
     <Folder Include="Properties\" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\DigitalData.EmailProfiler.Application\DigitalData.EmailProfiler.Application.csproj" />
+    <ProjectReference Include="..\DigitalData.EmailProfiler.Domain\DigitalData.EmailProfiler.Domain.csproj" />
+    <ProjectReference Include="..\DigitalData.EmailProfiler.Infrastructure\DigitalData.EmailProfiler.Infrastructure.csproj" />
+  </ItemGroup>
+
 </Project>

src/DigitalData.EmailProfiler.Application/DigitalData.EmailProfiler.Application.csproj

@@ -6,4 +6,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\DigitalData.EmailProfiler.Domain\DigitalData.EmailProfiler.Domain.csproj" />
+  </ItemGroup>
+
 </Project>

src/DigitalData.EmailProfiler.Infrastructure/DigitalData.EmailProfiler.Infrastructure.csproj

@@ -6,4 +6,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\DigitalData.EmailProfiler.Domain\DigitalData.EmailProfiler.Domain.csproj" />
+  </ItemGroup>
+
 </Project>

tests/DigitalData.EmailProfiler.Tests/DigitalData.EmailProfiler.Tests.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>net8.0</TargetFramework>
@@ -16,6 +16,13 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.5.3" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\DigitalData.EmailProfiler.API\DigitalData.EmailProfiler.API.csproj" />
+    <ProjectReference Include="..\..\src\DigitalData.EmailProfiler.Application\DigitalData.EmailProfiler.Application.csproj" />
+    <ProjectReference Include="..\..\src\DigitalData.EmailProfiler.Domain\DigitalData.EmailProfiler.Domain.csproj" />
+    <ProjectReference Include="..\..\src\DigitalData.EmailProfiler.Infrastructure\DigitalData.EmailProfiler.Infrastructure.csproj" />
+  </ItemGroup>
+
   <ItemGroup>
     <Using Include="Xunit" />
   </ItemGroup>