[http-client-csharp] Build discriminated base description from discriminated base type, not abstract modifier

[Copilot]

The discriminator "Please note this is the abstract base class..." description was only emitted when a model carried the Abstract declaration modifier. Whether a discriminated base type is modeled as abstract is an implementation detail — downstream emitters (e.g. provisioning) may not declare it abstract, causing the derived-classes note to be dropped.

Changes

• ModelProvider: Added IsDiscriminatedBaseType helper — a model with a discriminator property but no discriminator value.
• BuildDescription: Gates the derived-classes note on IsDiscriminatedBaseType instead of the Abstract modifier, decoupling documentation from abstractness.
• BuildDeclarationModifiers: Reuses the same helper for the abstract decision, keeping the base emitter's behavior unchanged.
• Tests: Added DiscriminatedBaseDescriptionIsBuiltEvenWhenNotAbstract, using a ModelProvider subclass that strips Abstract to mimic a downstream emitter and asserting the note is still built.

private bool IsDiscriminatedBaseType =>
    _inputModel.DiscriminatorProperty is not null && _inputModel.DiscriminatorValue is null;

• Fixes [http-client-csharp] discriminator message should be added when the type is a discriminated base instead of checking abstract #11397

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@11412

commit: eb5834f

[jorgerangel-msft]

@copilot ensure we regenerate all the test libraries and all tests are passing

[Copilot]

@copilot ensure we regenerate all the test libraries and all tests are passing

Regenerated all test libraries via pwsh ./eng/scripts/Generate.ps1 (Sample-TypeSpec + all Spector projects) — this produced no generated-file changes, since the base emitter still marks discriminated base types as abstract, so the emitted descriptions are unchanged. Verified all tests pass:

• Microsoft.TypeSpec.Generator.Tests: 1722 passed, 0 failed
• Microsoft.TypeSpec.Generator.ClientModel.Tests: 1513 passed, 0 failed

[jorgerangel-msft]

regen preview: Azure/azure-sdk-for-net#61407