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

[ArcturusZhang]

I think this condition is inaccurate.
we should be building the following description when there is a base type in a discriminated set.
the fact whether this model is abstract or not is implementation details.
in downstream emitters, they might not still model discriminated base type "abstract" (such as the provisioning emitter)

Originally posted by @ArcturusZhang in #10988

[github-actions]

🎯 Issue Triage

The C# emitter incorrectly conditions discriminator description generation on whether a model is abstract, when it should instead check whether the model is a discriminated base type (i.e., appears in a discriminated set). This distinction matters because downstream emitters (e.g., provisioning emitter) may not model discriminated base types as abstract.

Category: Bug

Area: emitter:client:csharp

⚠️ No reproduction provided. The issue references PR #10988 where the problem was identified in a code review comment.

🔍 Analysis

• The bug is in the logic that decides when to add a discriminator description/message for a model
• Current condition: checks isAbstract on the model
• Correct condition: should check if the model is a discriminated base (i.e., has discriminatedSubtypes or is referenced as a base in a discriminator relationship)
• Related: #10797 describes a similar problem where serialization methods are missing for discriminated base models that aren't modeled as abstract — both issues stem from over-reliance on the abstract flag rather than the discriminator relationship

📝 Suggested next steps

• Identify the code location in http-client-csharp where discriminator message generation is gated on isAbstract
• Change the condition to check whether the model is a discriminated base type instead
• Add test coverage for non-abstract discriminated base types (e.g., used by the provisioning emitter pattern)

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