Fix namespace for IServiceCollection extensions (#5620)

We generally put extension methods into the same namespace as the thing they're extending.

Author: stephentoub

src/Libraries/Microsoft.Extensions.AI/ChatCompletion/ChatClientBuilderServiceCollectionExtensions.cs

@@ -2,10 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.AI;
 using Microsoft.Shared.Diagnostics;
 
-namespace Microsoft.Extensions.AI;
+namespace Microsoft.Extensions.DependencyInjection;
 
 /// <summary>Provides extension methods for registering <see cref="IChatClient"/> with a <see cref="IServiceCollection"/>.</summary>
 public static class ChatClientBuilderServiceCollectionExtensions

src/Libraries/Microsoft.Extensions.AI/Embeddings/EmbeddingGeneratorBuilderServiceCollectionExtensions.cs

@@ -2,10 +2,10 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
-using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.AI;
 using Microsoft.Shared.Diagnostics;
 
-namespace Microsoft.Extensions.AI;
+namespace Microsoft.Extensions.DependencyInjection;
 
 /// <summary>Provides extension methods for registering <see cref="IEmbeddingGenerator{TInput, TEmbedding}"/> with a <see cref="IServiceCollection"/>.</summary>
 public static class EmbeddingGeneratorBuilderServiceCollectionExtensions