.Net: Prepare OpenApi model classes to changes in OpenApi.NET v2 SDK (#9603)

### Motivation, Context and Description
The new version, V2, of the
[OpenAPI.NET](microsoft/OpenAPI.NET#1906) SDK
comes with changes to its model classes. This PR updates the SK OpenAPI
model classes to match the SDK's ones. The goal is to reduce breaking
changes from the update.

Similar changes were done in this PR - [.Net: Rename OpenAPI model
classes](#9595) where
the `RestApiPayloadProperty.Type`, `RestApiParameter.Type`, and
`RestApiParameter.ArrayItemType` were made internal to simplify the
update to OpenAPI.NET v2, where their types have been changed.

Related task: #6884

Author: SergeyMenshykh

dotnet/src/Functions/Functions.OpenApi/Model/RestApiOperation.cs

@@ -34,12 +34,12 @@ public sealed class RestApiOperation
     /// <summary>
     /// The operation identifier.
     /// </summary>
-    public string Id { get; }
+    public string? Id { get; }
 
     /// <summary>
     /// The operation description.
     /// </summary>
-    public string Description { get; }
+    public string? Description { get; }
 
     /// <summary>
     /// The operation path.
@@ -59,7 +59,7 @@ public sealed class RestApiOperation
     /// <summary>
     /// The security requirements.
     /// </summary>
-    public IReadOnlyList<RestApiSecurityRequirement>? SecurityRequirements { get; }
+    public IReadOnlyList<RestApiSecurityRequirement> SecurityRequirements { get; }
 
     /// <summary>
     /// The operation parameters.
@@ -90,29 +90,29 @@ public sealed class RestApiOperation
     /// <param name="method">The operation method.</param>
     /// <param name="description">The operation description.</param>
     /// <param name="parameters">The operation parameters.</param>
-    /// <param name="payload">The operation payload.</param>
     /// <param name="responses">The operation responses.</param>
     /// <param name="securityRequirements">The operation security requirements.</param>
+    /// <param name="payload">The operation payload.</param>
     internal RestApiOperation(
-        string id,
+        string? id,
         IReadOnlyList<RestApiOperationServer> servers,
         string path,
         HttpMethod method,
-        string description,
+        string? description,
         IReadOnlyList<RestApiOperationParameter> parameters,
-        RestApiOperationPayload? payload = null,
-        IReadOnlyDictionary<string, RestApiOperationExpectedResponse>? responses = null,
-        IReadOnlyList<RestApiSecurityRequirement>? securityRequirements = null)
+        IReadOnlyDictionary<string, RestApiOperationExpectedResponse> responses,
+        IReadOnlyList<RestApiSecurityRequirement> securityRequirements,
+        RestApiOperationPayload? payload = null)
     {
         this.Id = id;
         this.Servers = servers;
         this.Path = path;
         this.Method = method;
         this.Description = description;
         this.Parameters = parameters;
-        this.Payload = payload;
-        this.Responses = responses ?? new Dictionary<string, RestApiOperationExpectedResponse>();
+        this.Responses = responses;
         this.SecurityRequirements = securityRequirements;
+        this.Payload = payload;
     }
 
     /// <summary>

dotnet/src/Functions/Functions.OpenApi/OpenApiKernelPluginFactory.cs

@@ -325,7 +325,7 @@ private static string ConvertOperationToValidFunctionName(RestApiOperation opera
     {
         if (!string.IsNullOrWhiteSpace(operation.Id))
         {
-            return ConvertOperationIdToValidFunctionName(operationId: operation.Id, logger: logger);
+            return ConvertOperationIdToValidFunctionName(operationId: operation.Id!, logger: logger);
         }
 
         // Tokenize operation path on forward and back slashes

dotnet/src/Functions/Functions.UnitTests/OpenApi/Extensions/RestApiOperationExtensionsTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Net.Http;
 using Microsoft.SemanticKernel.Plugins.OpenApi;
@@ -255,13 +256,15 @@ public void ItShouldSetAlternativeNameToParametersForPutAndPostOperation(string
     private static RestApiOperation CreateTestOperation(string method, RestApiOperationPayload? payload = null, Uri? url = null)
     {
         return new RestApiOperation(
-                    id: "fake-id",
-                    servers: [new(url?.AbsoluteUri)],
-                    path: "fake-path",
-                    method: new HttpMethod(method),
-                    description: "fake-description",
-                    parameters: [],
-                    payload: payload);
+            id: "fake-id",
+            servers: [new(url?.AbsoluteUri)],
+            path: "fake-path",
+            method: new HttpMethod(method),
+            description: "fake-description",
+            parameters: [],
+            responses: new Dictionary<string, RestApiOperationExpectedResponse>(),
+            securityRequirements: [],
+            payload: payload);
     }
 
     private static RestApiOperationPayload CreateTestJsonPayload()