Fix FieldClearMap service registration issue causing runtime error

[Copilot]

The FieldClearMap was causing a runtime error when users tried to use it in their field mapping configuration:

System.InvalidOperationException: No service for type 'MigrationTools.FieldMaps.AzureDevops.ObjectModel.FieldClearMap' has been registered.
          at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
          at MigrationTools.Tools.FieldMappingTool..ctor(IOptions`1 options, IServiceProvider services, ILogger`1 logger, ITelemetryLogger telemetry)

This occurred when users configured the field map in their migration settings:

{
  "FieldMapType": "FieldClearMap",
  "ApplyTo": ["Feature"],
  "targetField": "System.Description"
}

The issue was that while the FieldClearMap class was fully implemented and documented, it was missing from the dependency injection service registration in the AddMigrationToolServicesForClientLegacyAzureDevOpsObjectModel method.

Root Cause

The FieldMappingTool resolves field maps from the DI container using Services.GetRequiredService(type), but FieldClearMap was not registered as a transient service like other field maps.

Changes

• Added context.AddTransient<FieldClearMap>(); to the service registration in ServiceCollectionExtensions.cs
• Added test coverage for FieldClearMapOptions configuration
• Added service registration test to verify DI container resolution

Impact

Users can now successfully use the FieldClearMap functionality that was already documented and implemented but previously inaccessible due to the missing service registration.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[Bug]:</issue_title>
<issue_description>### Version

• I confirm that I am using the latest version

Source Version

Azure DevOps Service

Target Version

Azure DevOps Service

Relevant configuration

{
            "FieldMapType": "FieldClearMap",
            "ApplyTo": [
              "Feature"
            ],
            "targetField": "System.Description"
          }

Relevant log output

Unhandled exception!
System.InvalidOperationException: No service for type 'MigrationTools.FieldMaps.AzureDevops.ObjectModel.FieldClearMap' has been registered.
          at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType)
          at MigrationTools.Tools.FieldMappingTool..ctor(IOptions`1 options, IServiceProvider services, ILogger`1 logger, ITelemetryLogger telemetry) in D:\a\1\s\src\MigrationTools\Tools\FieldMappingTool.cs:line 48

What happened?

I am trying to use the FieldClearMap to clear the Description field for migrated work items. I get the error 'No service for type 'MigrationTools.FieldMaps.AzureDevops.ObjectModel.FieldClearMap' has been registered.' and then the migrator quits without migrating the work item.

Debug in Visual Studio

• Visual Studio Debug</issue_description>

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

Fixes #3002

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