[Azure Mgmt Generator] Add mocking documentation to extension methods

[Copilot]

Description

Extension methods in the Azure Management Generator were missing XML documentation to guide users on mocking. The previous generator included this documentation in an <item> block within the <summary> tag, but the new TypeSpec-based generator omitted it.

Changes

• ExtensionProvider.cs: Modified BuildRedirectMethod to generate mocking documentation for all extension methods
  • Changed signature to accept MockableResourceProvider instead of CSharpType to access mockable type metadata
  • Added AddMockingDocumentation method to inject <item> block with mocking instructions
  • Added BuildMockingDescription helper to construct method references with parameter types

Generated Output

Extension methods now include mocking guidance:

/// <summary>
/// Gets an object representing a <see cref="FooResource"/> along with the instance operations...
/// <item>
/// <term> Mocking. </term>
/// <description> To mock this method, please mock <see cref="MockableFooArmClient.GetFooResource(ResourceIdentifier)"/> instead. </description>
/// </item>
/// </summary>
public static FooResource GetFooResource(this ArmClient client, ResourceIdentifier id)

Applies to all ARM extension methods across ArmClient, ResourceGroupResource, SubscriptionResource, TenantResource, and ManagementGroupResource.

---

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• Pull request includes test coverage for the included changes.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Azure Mgmt Generator] Methods in extension classes missing mocking related doc</issue_title>
<issue_description>
This is how it should look like on a method generated by our previous generator:

        /// <summary>
        /// Gets an object representing a <see cref="ComputeLimitSharedLimitResource" /> along with the instance operations that can be performed on it but with no data.
        /// You can use <see cref="ComputeLimitSharedLimitResource.CreateResourceIdentifier" /> to create a <see cref="ComputeLimitSharedLimitResource" /> <see cref="ResourceIdentifier" /> from its components.
        /// <item>
        /// <term>Mocking</term>
        /// <description>To mock this method, please mock <see cref="MockableComputeLimitArmClient.GetComputeLimitSharedLimitResource(ResourceIdentifier)"/> instead.</description>
        /// </item>
        /// </summary>
        /// <param name="client"> The <see cref="ArmClient" /> instance the method will execute against. </param>
        /// <param name="id"> The resource ID of the resource to get. </param>
        /// <exception cref="ArgumentNullException"> <paramref name="client"/> is null. </exception>
        /// <returns> Returns a <see cref="ComputeLimitSharedLimitResource"/> object. </returns>
        public static ComputeLimitSharedLimitResource GetComputeLimitSharedLimitResource(this ArmClient client, ResourceIdentifier id)
        {
            Argument.AssertNotNull(client, nameof(client));

            return GetMockableComputeLimitArmClient(client).GetComputeLimitSharedLimitResource(id);
        }
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes [Azure Mgmt Generator] Methods in extension classes missing mocking related doc #53968

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Pull request overview

This PR adds mocking documentation to extension methods generated by the Azure Management Generator. Previously, these extension methods lacked guidance on how users should mock them for testing purposes. The implementation adds an <item> block within the XML <summary> tag to direct users to mock the corresponding method in the mockable resource provider instead.

Key Changes:

• Modified BuildRedirectMethod to accept MockableResourceProvider instead of just CSharpType to access mockable type metadata
• Added AddMockingDocumentation method to inject mocking instructions into method XML documentation
• Added BuildMockingDescription helper to construct method references with parameter type information

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 3 comments.

File	Description
eng/packages/http-client-csharp-mgmt/generator/Azure.Generator.Management/src/Providers/ExtensionProvider.cs	Modified to generate mocking documentation for extension methods, including new methods to build and inject XML documentation
eng/packages/http-client-csharp-mgmt/generator/TestProjects/Local/Mgmt-TypeSpec/src/Generated/Extensions/AzureGeneratorMgmtTypeSpecTestsExtensions.cs	Test output showing generated mocking documentation added to all extension methods across various ARM resource types