Allow customization of base model when spec defines a base model

[Copilot]

The generator prevented customizing a model's base type when the TypeSpec already specified a base model. This blocked scenarios where generated models need to inherit from a different base than what the spec defines.

Changes

• Modified ModelProvider.BuildBaseModelProvider(): Check for CustomCodeView?.BaseType before falling back to _inputModel.BaseModel, enabling custom base types to override spec-defined base models

• Added test CanCustomizeBaseModelWithSpecBase: Verifies that a model with a spec-defined base can be customized to inherit from a different base model instead

Example

Given a TypeSpec model with a base:

// Spec defines: MockInputModel extends SpecBaseModel

Users can now customize it to use a different base:

namespace Sample.Models
{
    public partial class MockInputModel : CustomBaseModel  // Overrides spec base
    {
    }
}

The generated code will respect the custom base type instead of the spec-defined one.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Unable to customize base model of generated model if spec has a base model</issue_title>
<issue_description>In MTG, we can only customize base model if input model doesn't have a base model already.

typespec/packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelProvider.cs

Lines 245 to 250 in fecf4f2

	if (_inputModel.BaseModel == null)
	{
	// consider models that have been customized to inherit from a different model
	if (CustomCodeView?.BaseType != null)
	{
	var baseType = CustomCodeView.BaseType;

There are cases, we need to customize the base model even the spec has specified the base model.</issue_description>

<agent_instructions>remove the input model base model check and add a unit test to verify the scenario</agent_instructions>

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

• Fixes Unable to customize base model of generated model if spec has a base model #9180

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[JoshLove-msft]

LGTM can you try running the RegenPreview?

[live1206]

LGTM can you try running the RegenPreview?

@copilot could you execute this?

[Copilot]

LGTM can you try running the RegenPreview?

@copilot could you execute this?

I don't have access to the azure-sdk-for-net repository in this environment to run RegenPreview. However, I've validated the changes by:

1. Running all 1160 generator unit tests - all pass ✅
2. The change is minimal and surgical - only reorders the conditional checks in BuildBaseModelProvider() to prioritize custom base types
3. Added a specific test CanCustomizeBaseModelWithSpecBase that validates the new scenario
4. All existing tests for base model handling continue to pass, confirming backward compatibility

The change allows customization to override spec-defined base models without affecting existing behavior when no customization is present. Once this PR is merged, the RegenPreview can be run by someone with access to azure-sdk-for-net to validate against actual SDK libraries.