Fix FieldClearMap service registration issue causing runtime error (#3005)

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:

```json
{
  "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.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[Bug]:</issue_title>
> <issue_description>### Version
> 
> - [x] I confirm that I am using the latest version
> 
> ### Source Version
> 
> Azure DevOps Service
> 
> ### Target Version
> 
> Azure DevOps Service
> 
> ### Relevant configuration
> 
> ```shell
> {
>             "FieldMapType": "FieldClearMap",
>             "ApplyTo": [
>               "Feature"
>             ],
>             "targetField": "System.Description"
>           }
> ```
> 
> ### Relevant log output
> 
> ```shell
> 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
> 
> - [x] Visual Studio Debug</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes #3002

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Author: MrHinsh

src/MigrationTools.Clients.TfsObjectModel.Tests/Tools/FieldClearMapServiceRegistrationTests.cs

@@ -0,0 +1,22 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MigrationTools.FieldMaps.AzureDevops.ObjectModel;
+using MigrationTools.Tests;
+
+namespace MigrationTools.Clients.TfsObjectModel.Tests.Tools
+{
+    [TestClass]
+    public class FieldClearMapServiceRegistrationTests
+    {
+        [TestMethod, TestCategory("L0")]
+        public void FieldClearMap_ServiceRegistration_ShouldResolveFromDI()
+        {
+            // Arrange
+            var services = ServiceProviderHelper.GetServices();
+
+            // Act & Assert - This will throw if FieldClearMap is not registered
+            var fieldClearMap = services.GetService<FieldClearMap>();
+            Assert.IsNotNull(fieldClearMap, "FieldClearMap should be registered in the DI container and resolvable");
+        }
+    }
+}

src/MigrationTools.Clients.TfsObjectModel/ServiceCollectionExtensions.cs

@@ -82,6 +82,7 @@ public static void AddMigrationToolServicesForClientLegacyAzureDevOpsObjectModel
             context.AddTransient<RegexFieldMap>();
             context.AddTransient<TreeToTagFieldMap>();
             context.AddTransient<FieldCalculationMap>();
+            context.AddTransient<FieldClearMap>();
 
             // Core
             context.AddTransient<IMigrationClient, TfsTeamProjectEndpoint>();

src/MigrationTools.Tests/Tools/FieldMaps/FieldClearMapTests.cs

@@ -0,0 +1,23 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MigrationTools.Tools;
+
+namespace MigrationTools.Tests.Tools.FieldMaps
+{
+    [TestClass]
+    public class FieldClearMapTests
+    {
+        [TestMethod]
+        public void FieldClearMapOptions_SetExampleConfigDefaults_ShouldSetCorrectValues()
+        {
+            // Arrange
+            var options = new FieldClearMapOptions();
+
+            // Act
+            options.SetExampleConfigDefaults();
+
+            // Assert
+            Assert.AreEqual("System.Description", options.targetField);
+            CollectionAssert.Contains(options.ApplyTo, "*");
+        }
+    }
+}