Fix malformed namespace generation for unresolved types in customization code

[Copilot]

Fix for Malformed Namespace Generation in Customization Code

• Understand root cause: Roslyn cannot resolve generated types in customization code, leading to malformed namespaces
• Move fix from TypeSymbolExtensions to CSharpTypeExtensions.EnsureNamespace per code review feedback
• Enhanced EnsureNamespace to look up types by name when spec property is null
• Handle edge case where types are renamed with CodeGenType attribute
• Add TypeProvidersByName dictionary for efficient name-based lookup
• Remove redundant code and unnecessary variables
• Add comprehensive tests validating the fix
• Validate with existing test suite - all 2260 tests pass (including 2 new tests)

Changes Made

Modified: TypeFactory.cs

• Added TypeProvidersByName dictionary mapping C# type names to TypeProviders for efficient O(1) lookup
• Populated dictionary when adding types to CSharpTypeMap in CreateModel and CreateEnum

Modified: CSharpTypeExtensions.cs

• Enhanced EnsureNamespace method to handle custom properties without corresponding spec properties
• Added TryFindCSharpTypeByName with inline TypeFactory access for cleaner code
• Returns properly qualified CSharpType by resolving through TypeProvidersByName

Added Tests: ModelCustomizationTests.cs

• CanAddPropertyReferencingGeneratedType: Validates custom property IList<Bar> gets proper namespace "Sample.Models"
• CanAddPropertyReferencingRenamedGeneratedType: Validates custom property referencing CodeGenType renamed type gets proper namespace
• Both tests confirm namespaces are correctly resolved instead of being empty or malformed

Testing

All tests pass, confirming the fix resolves malformed namespace generation for customization code.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[http-client-csharp] malformed code is generated on some kind of customization code</issue_title>
<issue_description>Sometimes, in customized code, roslyn cannot analyze the correct namespace of some symbols (or maybe not?), and code like this is generated:

We need to fix this.
The above code was generated when we have the following customization:

To reproduce:
Prepare typespec:

model Foo {
	bar?: Bar;
}

model Bar {
	b?: string;
}

In the generated code, add the following:

[CodeGenSerialization(nameof(Bars), "bars"]
public partial class Foo
{
	public IList<Bar> Bars {get;}
}

then generate the code.</issue_description>

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

@JoshLove-msft @ArcturusZhang can you please add repro instructions? @ArcturusZhang > [@ArcturusZhang](https://github.com/ArcturusZhang) can you please add repro instructions?

added some reproduction steps.</comment_new>
<comment_new>@jorgerangel-msft
The root cause of this may be that we are not successfully reading the namespace of "non-primitive" generated types in the symbols therefore when we write them out, there are no namespaces.</comment_new>

• Fixes [http-client-csharp] malformed code is generated on some kind of customization code #9403

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[JoshLove-msft]

@copilot please add tests to ModelCustomizationTests that validates the case where a custom partial type adds a new property that references a generated type. The generated constructor for this model should represent the property as a parameter correctly, i.e. the namespace should match the generated type namespace. Add a similar test case where the generated type referenced in the custom property is customized to use a different name using CodeGenType.

[Copilot]

@copilot please add tests to ModelCustomizationTests that validates the case where a custom partial type adds a new property that references a generated type. The generated constructor for this model ...

Added two test cases to ModelCustomizationTests:

1. CanAddPropertyReferencingGeneratedType - validates custom property IList<Bar> has proper namespace in constructor parameters
2. CanAddPropertyReferencingRenamedGeneratedType - validates custom property referencing a CodeGenType renamed type (RenamedBar for TypeSpec Bar) has proper namespace

Both tests confirm the namespace is correctly resolved as "Sample.Models" instead of being empty or malformed. Changes in commit 649f0d2.