Validate OpenAPI documents create valid C# clients (#3686)

* Add code generation integration tests

Initial infrastructure for integration tests that validate OpenAPI documents can generate valid C# clients using Kiota and NSwag.

* Refactor client generator code

Move to its own class to slim down the test.

* Extend Pet Store test cases

Add test cases for Swagger 2.0 and YAML, where relevant.

* Generate clients from snapshots

Generate client code from snapshot files and running applications instead of Pet Store.

* Enable nullable

Enable nullable annotations in the generated client code.

* Deduplicate tests

Just validate the snapshots, not the applications as well (which are used to generate the snapshots).

* Refactor switch statement

Refactor to make the lines less long.

Author: martincostello

.config/dotnet-tools.json

@@ -22,6 +22,13 @@
         "meziantou.validate-nuget-package"
       ],
       "rollForward": false
+    },
+    "microsoft.openapi.kiota": {
+      "version": "1.29.0",
+      "commands": [
+        "kiota"
+      ],
+      "rollForward": false
     }
   }
 }

.github/workflows/build.yml

@@ -88,6 +88,11 @@ jobs:
       uses: actions/setup-dotnet@2016bd2012dba4e32de620c46fe006a3ac9f0602 # v5.0.1
       id: setup-dotnet
 
+    - name: Install .NET tools
+      shell: pwsh
+      run: |
+        dotnet tool restore
+
     - name: Build, Package and Test
       id: build
       shell: pwsh

Directory.Packages.props

@@ -13,13 +13,17 @@
     <PackageVersion Include="Microsoft.AspNetCore.Mvc.Versioning" Version="5.1.0" />
     <PackageVersion Include="Microsoft.AspNetCore.Mvc.Versioning.ApiExplorer" Version="5.1.0" />
     <PackageVersion Include="Microsoft.AspNetCore.TestHost" Version="8.0.22" />
+    <PackageVersion Include="Microsoft.Build" Version="18.0.2" />
     <PackageVersion Include="Microsoft.CodeAnalysis.PublicApiAnalyzers" Version="4.14.0" />
+    <PackageVersion Include="Microsoft.Kiota.Bundle" Version="1.21.0" />
     <PackageVersion Include="Microsoft.OpenApi" Version="2.3.9" />
     <PackageVersion Include="Microsoft.OpenApi.YamlReader" Version="2.3.9" />
     <PackageVersion Include="Microsoft.Playwright" Version="1.56.0" />
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
+    <PackageVersion Include="MSBuild.ProjectCreation" Version="17.0.1" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.4" />
     <PackageVersion Include="NSubstitute" Version="5.3.0" />
+    <PackageVersion Include="NSwag.CodeGeneration.CSharp" Version="14.6.3" />
     <PackageVersion Include="NSwag.MSBuild" Version="14.6.3" />
     <PackageVersion Include="ReportGenerator" Version="5.5.0" />
     <PackageVersion Include="Verify.XunitV3" Version="31.7.2" />