Generate CodeGen customization attributes in fixed namespace (#9122)

- [x] Understand the issue: CodeGen customization attributes should be
generated in a fixed namespace to avoid API compatibility failures
- [x] Identify the suggested namespace from Azure SDK issue #54144:
`Microsoft.TypeSpec.Generator.Customizations`
- [x] Add `CustomizationAttributeNamespace` constant to
`CodeModelGenerator` class (changed to `internal` per review feedback)
- [x] Override `BuildNamespace()` in CodeGen attribute definition
classes to return the fixed namespace
- [x] Update test data files to use the new namespace
- [x] Update expected test strings in `ClientCustomizationTests`
- [x] Update custom code files in Sample-TypeSpec to import the new
namespace
- [x] Update generated attribute files in Sample-TypeSpec to use the new
namespace
- [x] Run tests to validate changes (all pass)
- [x] Request code review (passed with no comments)
- [x] Address review feedback: Changed `CustomizationAttributeNamespace`
from `public` to `internal`

## Security Summary
No security vulnerabilities were introduced. The changes only modify
namespace declarations and add using directives - no new code paths, no
external inputs, and no changes to security-sensitive logic.

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Generate CodeGen customization attributes in fixed
namespace</issue_title>
> <issue_description>See
Azure/azure-sdk-for-net#54144 for more
context.</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #9121

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: JoshLove-msft <54595583+JoshLove-msft@users.noreply.github.com>

Author: Copilot

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ClientProviders/TestData/ClientProviderCustomizationTests/CanRemoveCachingField/CanRemoveCachingField.cs

@@ -1,6 +1,7 @@
 
 using System;
 using SampleTypeSpec;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/ClientProviders/TestData/ClientProviderCustomizationTests/CanRenameSubClient/CanRenameSubClient.cs

@@ -1,4 +1,5 @@
 using SampleTypeSpec;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/Definitions/TestData/Classifier2xxAnd4xxDefinitionTests/TestGetTypeCustomNamespace/TestGetTypeCustomNamespace.cs

@@ -4,6 +4,7 @@
 using System.ClientModel.Primitives;
 using System.Threading.Tasks;
 using SampleTypeSpec;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Custom;
 

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/EnumProvider/TestData/SerializationCustomizationTests/CanChangeEnumMemberName/MockInputModel.cs

@@ -1,6 +1,7 @@
 #nullable disable
 
 using SampleTypeSpec;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Models
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/PropagateCustomizedDynamicListProperty/DynamicModel.cs

@@ -1,5 +1,6 @@
 using SampleTypeSpec;
 using System.Collections.Generic;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Models
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/PropagateCustomizedDynamicListPropertyMixed/DynamicModel.cs

@@ -1,5 +1,6 @@
 using SampleTypeSpec;
 using System.Collections.Generic;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Models
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/PropagateCustomizedDynamicProperty/DynamicModel.cs

@@ -1,5 +1,7 @@
 using SampleTypeSpec;
 
+using Microsoft.TypeSpec.Generator.Customizations;
+
 namespace Sample.Models
 {
     public partial class DynamicModel

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/PropagateCustomizedDynamicPropertyMixed/DynamicModel.cs

@@ -1,5 +1,7 @@
 using SampleTypeSpec;
 
+using Microsoft.TypeSpec.Generator.Customizations;
+
 namespace Sample.Models
 {
     public partial class DynamicModel

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/DynamicModelSerializationTests/WriteReadOnlySpanProperty/DynamicModel.cs

@@ -3,6 +3,7 @@
 using SampleTypeSpec;
 using System;
 using System.Diagnostics.CodeAnalysis;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Models
 {

packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator.ClientModel/test/Providers/MrwSerializationTypeDefinitions/TestData/ModelCustomizationTests/CanChangePropertyName/MockInputModel.cs

@@ -1,6 +1,7 @@
 #nullable disable
 
 using SampleTypeSpec;
+using Microsoft.TypeSpec.Generator.Customizations;
 
 namespace Sample.Models
 {