Use MRW for ManagedServiceIdentity serialization

[live1206]

This PR is to release a beta package of Azure.ResourceManager to apply with the code change from generator in Azure/autorest.csharp#5347

[Copilot]

Pull Request Overview

This PR updates the serialization logic for ManagedServiceIdentity to support the new MRW approach and the V3 format while also updating various test assets and related test code. Key changes include:

• Updates to test asset JSON files (removing duplicate or extra closing brackets and fields).
• Modifications in test helper methods and ManagedServiceIdentity serialization/deserialization logic to support V3 formatting.
• Adjustments in test utilities and removal of obsolete tests.

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sdk/resourcemanager/Azure.ResourceManager/tests/Unit/TestAssets/Identity/*.json	Removed extra or duplicate fields to correctly represent identity JSON structures
sdk/resourcemanager/Azure.ResourceManager/tests/Unit/ManagedServiceIdentityTest.cs	Updated deserialization helper to support MRW and included string normalization changes
sdk/resourcemanager/Azure.ResourceManager/tests/Unit/JsonAsserts.cs	Adjusted AssertConverterSerialization to work with IJsonModel and ModelReaderWriterOptions
sdk/resourcemanager/Azure.ResourceManager/tests/Scenario/PolicyAssignmentCollectionTests.cs	Removed obsolete ManagedIdentity test cases
sdk/resourcemanager/Azure.ResourceManager/src/Common/Custom/Models/ManagedServiceIdentity.Serialization.cs	Enhanced V3 handling and improved null handling for user assigned identities
sdk/resourcemanager/Azure.ResourceManager/src/Common/Custom/Models/AzureResourceManagerContext.cs	Added builder registration for ManagedServiceIdentity