Add buildless failed diagnostic

csharp/ql/integration-tests/all-platforms/standalone_failed/diagnostics.expected

@@ -0,0 +1,28 @@
+{
+  "markdownMessage": "C# analysis with build-mode 'none' failed.",
+  "severity": "error",
+  "source": {
+    "extractorName": "csharp",
+    "id": "csharp/autobuilder/buildless/failed",
+    "name": "C# analysis with build-mode 'none' failed"
+  },
+  "visibility": {
+    "cliSummaryTable": true,
+    "statusPage": true,
+    "telemetry": true
+  }
+}
+{
+  "markdownMessage": "C# with build-mode set to 'none'. This means that all C# source in the working directory will be scanned, with build tools, such as Nuget and Dotnet CLIs, only contributing information about external dependencies.",
+  "severity": "note",
+  "source": {
+    "extractorName": "csharp",
+    "id": "csharp/autobuilder/buildless/mode-active",
+    "name": "C# with build-mode set to 'none'"
+  },
+  "visibility": {
+    "cliSummaryTable": true,
+    "statusPage": true,
+    "telemetry": true
+  }
+}

csharp/ql/integration-tests/all-platforms/standalone_failed/standalone.csproj

@@ -0,0 +1,14 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+ <Target Name="DeleteBinObjFolders" BeforeTargets="Clean">
+   <RemoveDir Directories=".\bin" />
+   <RemoveDir Directories=".\obj" />
+ </Target>
+</Project>

csharp/ql/integration-tests/all-platforms/standalone_failed/test.py

@@ -0,0 +1,6 @@
+from create_database_utils import *
+from diagnostics_test_utils import *
+
+run_codeql_database_create([], db=None, lang="csharp", extra_args=["--build-mode=none"], runFunction=runUnsuccessfully)
+
+check_diagnostics()