feat: add automated unit test pipeline with code coverage

- Add GitHub Actions workflow for running 253 unit tests on every push/PR
- Integrate Coverlet for code coverage collection and reporting
- Generate professional markdown test reports via ReportGenerator
- Save test reports to docs/unit-test-report.md with auto-commit
- Update MessagePack to 3.1.0 for security patches
- Add test status badge to README
- Tests: All 253 tests passing with coverage metrics

Author: sjefvanleeuwen

.github/workflows/unit-tests.yml

@@ -30,7 +30,7 @@ jobs:
         run: dotnet build tests/SharpGraph.Tests/SharpGraph.Tests.csproj --configuration Debug
 
       - name: Run Tests
-        run: dotnet test tests/SharpGraph.Tests/SharpGraph.Tests.csproj --configuration Debug --no-build --verbosity normal --logger "trx;LogFileName=../../test-results.trx" --collect:"XPlat Code Coverage" --logger "console;verbosity=detailed"
+        run: dotnet test tests/SharpGraph.Tests/SharpGraph.Tests.csproj --configuration Debug --no-build --verbosity normal --logger "trx;LogFileName=../../test-results.trx" --logger "console;verbosity=detailed" /p:CollectCoverage=true /p:CoverletOutputFormat=cobertura /p:CoverletOutput=..\..\coverage.cobertura.xml
 
       - name: Install Report Generator
         run: dotnet tool install --global dotnet-reportgenerator-globaltool

tests/SharpGraph.Tests/SharpGraph.Tests.csproj

@@ -15,6 +15,10 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="6.0.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup>