Prevent C# model factories from instantiating abstract unknown variants

[Copilot]

Back-compat model factories must return a concrete Unknown* variant for abstract models or omit the method when none is available. Instantiating an abstract type produces invalid C# (CS0144).

• Factory selection: Exclude abstract unknown variants from the shared selector used by regular and back-compat factories.
• Regression coverage: Cover direct construction when no current overload exists, missing unknown variants, and unknown variants customized as abstract.

• Fixes Back Compat Model Factory Method Should Not Instantiate Abstract Model Types #11877

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npm i https://pkg.pr.new/@typespec/http-client-csharp@11878

commit: d20e4de

[Copilot]

🟢 Approval recommended

The change is narrowly scoped to constructor target selection, and the added regression tests cover the key failure modes described in the issue.

Pull request overview

This PR fixes C# model factory generation to avoid instantiating abstract model types when producing factory methods (including back-compat overloads), which can otherwise emit invalid C# (CS0144). It aligns factory selection with the requirement that abstract models should be constructed via a concrete Unknown* discriminator variant when available, or skipped when not.

Changes:

• Updated the shared model-to-instantiate selector to exclude abstract Unknown* discriminator variants.
• Added regression tests for (1) back-compat method generation when there is no current overload and the return type is abstract, and (2) skipping generation when no concrete unknown variant is available (including when the unknown variant is customized as abstract).

File summaries

File	Description
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/src/Providers/ModelFactoryProvider.cs	Ensures abstract models only select a concrete unknown discriminator model for instantiation; otherwise no model is selected.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/ModelFactoryProviderTests.cs	Adds regression coverage for abstract return types in back-compat factories, including skip behavior when no concrete unknown exists.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeWithoutCurrentOverload/SampleNamespaceModelFactory.cs	Adds last-contract source for the “no current overload” back-compat scenario.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeWithoutCurrentOverload.cs	Adds expected emitted model factory output validating concrete unknown instantiation.
packages/http-client-csharp/generator/Microsoft.TypeSpec.Generator/test/Providers/ModelFactories/TestData/ModelFactoryProviderTests/BackCompatibility_AbstractReturnTypeWithoutConcreteUnknownModelIsSkipped/AbstractModel.cs	Adds customization source making the unknown variant abstract to validate the “skip method” behavior.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Lite

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