Fix CI workflow: use .sln instead of .slnx

Corrects the test command to use the correct solution file extension,
ensuring tests run properly on all target frameworks (.NET 8, 9, 10).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ANcpLua

.github/workflows/tests.yml

@@ -32,7 +32,7 @@ jobs:
 
       - name: Run tests with coverage
         run: |
-          dotnet test SWEN3.Paperless.RabbitMq.slnx --no-build --configuration Release \
+          dotnet test SWEN3.Paperless.RabbitMq.sln --no-build --configuration Release \
             --collect:"XPlat Code Coverage" \
             --logger:"console;verbosity=detailed"