Improve error message when all discovered projects are test projects

Fixes #3065

Update error handling to provide a clear message when only test projects are found.

---

For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/stryker-mutator/stryker-net/issues/3065?shareId=XXXX-XXXX-XXXX-XXXX).

src/Stryker.Abstractions/Exceptions/InvalidProjectsException.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace Stryker.Abstractions.Exceptions;
+
+/// <summary>
+/// Represents error when no test projects are found in the solution or configured for stryker.
+/// </summary>
+public class InvalidProjectsException : Exception
+{
+    public InvalidProjectsException(string message)
+        : base(message)
+    {
+    }
+
+    public static InvalidProjectsException NoTestProjectsFound()
+    {
+        return new InvalidProjectsException("No test projects found. Please add a test project to your solution or fix your stryker config.");
+    }
+
+    public static InvalidProjectsException OnlyTestProjectsFound()
+    {
+        return new InvalidProjectsException("Only test projects found. Please ensure that your solution contains non-test projects.");
+    }
+}

src/Stryker.CLI/Stryker.CLI/Program.cs

@@ -13,7 +13,7 @@ public static int Main(string[] args)
             var app = new StrykerCli();
             return app.Run(args);
         }
-        catch (NoTestProjectsException exception)
+        catch (InvalidProjectsException exception)
         {
             AnsiConsole.WriteLine(exception.Message);
             return ExitCodes.Success;

src/Stryker.Core/Stryker.Core.UnitTest/StrykerRunnerTests.cs

@@ -181,7 +181,50 @@ public void ShouldThrow_WhenNoProjectsFound()
 
         var target = new StrykerRunner(reporterFactory: reporterFactoryMock.Object);
 
-        Should.Throw<NoTestProjectsException>(() => target.RunMutationTest(inputsMock.Object, new LoggerFactory(), projectOrchestratorMock.Object));
+        Should.Throw<InvalidProjectsException>(() => target.RunMutationTest(inputsMock.Object, new LoggerFactory(), projectOrchestratorMock.Object));
+
+        reporterMock.Verify(x => x.OnStartMutantTestRun(It.IsAny<IList<IMutant>>()), Times.Never);
+        reporterMock.Verify(x => x.OnMutantTested(It.IsAny<IMutant>()), Times.Never);
+        reporterMock.Verify(x => x.OnAllMutantsTested(It.IsAny<IReadOnlyProjectComponent>(), It.IsAny<TestProjectsInfo>()), Times.Never);
+    }
+
+    [TestMethod]
+    public void ShouldThrow_WhenOnlyTestProjectsFound()
+    {
+        var projectOrchestratorMock = new Mock<IProjectOrchestrator>(MockBehavior.Strict);
+        var reporterFactoryMock = new Mock<IReporterFactory>(MockBehavior.Strict);
+        var reporterMock = new Mock<IReporter>(MockBehavior.Strict);
+        var inputsMock = new Mock<IStrykerInputs>(MockBehavior.Strict);
+        var fileSystemMock = new MockFileSystem();
+
+        var folder = new CsharpFolderComposite();
+        folder.Add(new CsharpFileLeaf
+        {
+            Mutants = new Collection<IMutant>() { new Mutant() { Id = 1, ResultStatus = MutantStatus.Ignored } }
+        });
+        var mutationTestInput = new MutationTestInput()
+        {
+            SourceProjectInfo = new SourceProjectInfo()
+            {
+                ProjectContents = folder
+            }
+        };
+
+        inputsMock.Setup(x => x.ValidateAll()).Returns(new StrykerOptions
+        {
+            ProjectPath = "C:/test",
+            OptimizationMode = OptimizationModes.None,
+            LogOptions = new LogOptions()
+        });
+
+        projectOrchestratorMock.Setup(x => x.MutateProjects(It.IsAny<StrykerOptions>(), It.IsAny<IReporter>(), It.IsAny<ITestRunner>()))
+            .Returns(new List<IMutationTestProcess>() { });
+
+        reporterFactoryMock.Setup(x => x.Create(It.IsAny<StrykerOptions>(), It.IsAny<IGitInfoProvider>())).Returns(reporterMock.Object);
+
+        var target = new StrykerRunner(reporterFactory: reporterFactoryMock.Object);
+
+        Should.Throw<InvalidProjectsException>(() => target.RunMutationTest(inputsMock.Object, new LoggerFactory(), projectOrchestratorMock.Object));
 
         reporterMock.Verify(x => x.OnStartMutantTestRun(It.IsAny<IList<IMutant>>()), Times.Never);
         reporterMock.Verify(x => x.OnMutantTested(It.IsAny<IMutant>()), Times.Never);

src/Stryker.Core/Stryker.Core/StrykerRunner.cs

@@ -181,7 +181,12 @@ private IReadOnlyProjectComponent AddRootFolderIfMultiProject(IEnumerable<IReadO
     {
         if (!projectComponents.Any())
         {
-            throw new NoTestProjectsException();
+            throw InvalidProjectsException.NoTestProjectsFound();
+        }
+
+        if (projectComponents.All(pc => pc is TestProject))
+        {
+            throw InvalidProjectsException.OnlyTestProjectsFound();
         }
 
         if (projectComponents.Count() > 1)