Fix --no-build flag error message inconsistency for non-existent project files

Co-authored-by: marcpopMSFT <[email protected]>

Author: Copilot

src/Cli/dotnet/Commands/Run/RunCommand.cs

@@ -459,6 +459,12 @@ private static void ThrowUnableToRunError(ProjectInstance project)
             ? TryFindSingleProjectInDirectory(projectFileOrDirectoryPath)
             : projectFileOrDirectoryPath;
 
+        // Check if the project file actually exists when it's specified as a direct file path
+        if (projectFilePath is not null && !emptyProjectOption && !File.Exists(projectFilePath))
+        {
+            throw new GracefulException(CliCommandStrings.CmdNonExistentFileErrorDescription, projectFilePath);
+        }
+
         // If no project exists in the directory and no --project was given,
         // try to resolve an entry-point file instead.
         entryPointFilePath = projectFilePath is null && emptyProjectOption

test/dotnet.Tests/CommandTests/Run/GivenDotnetRunBuildsCsProj.cs

@@ -987,5 +987,33 @@ public void EnvOptionNotAppliedToBuild()
                .And
                .HaveStdOutContaining("env: Configuration=XYZ");
         }
+
+        [Fact]
+        public void ItProvidesConsistentErrorMessageWhenProjectFileDoesNotExistWithNoBuild()
+        {
+            var tempDir = _testAssetsManager.CreateTestDirectory();
+            var nonExistentProject = Path.Combine(tempDir.Path, "nonexistent.csproj");
+
+            var result = new DotnetCommand(Log, "run")
+                .WithWorkingDirectory(tempDir.Path)
+                .Execute("--project", nonExistentProject, "--no-build");
+
+            result.Should().Fail();
+            if (!TestContext.IsLocalized())
+            {
+                // After the fix, we should get a clear error message about the file not existing
+                var stderr = result.StdErr;
+                
+                Log.WriteLine($"Actual stderr: '{stderr}'");
+                
+                // Should provide a clear error message about the project file not existing
+                var hasExpectedErrorMessage = stderr.Contains("does not exist") || 
+                                              stderr.Contains("not found") || 
+                                              stderr.Contains("cannot find") || 
+                                              stderr.Contains("could not find");
+                                              
+                hasExpectedErrorMessage.Should().BeTrue($"Expected error message to clearly indicate file doesn't exist, but got: {stderr}");
+            }
+        }
     }
 }