C# generator: omit removed types from BinaryData union documentation

[live1206]

Description

When a BinaryData property carries union-item metadata, its generated XML documentation can list a generated type that the provider reference map later removes. The current writer avoids an invalid cref by rendering the removed type as <c> text, but the resulting documentation may still advertise a type that users cannot access.

Constraint

PropertyDescriptionBuilder.CreateBinaryDataExtraDescription and GetUnionTypes run while providers and property descriptions are being built. The provider reference map determines removal only afterward, once all providers and their dependencies have been built and analyzed. Therefore, checking ProviderReferenceMapAnalyzer.IsRemovedTypeReference while constructing the description is too early and cannot reliably filter the item.

Expected fix

Introduce a generic write-time or post-analysis mechanism that omits removed union-item types from generated BinaryData documentation while preserving valid union descriptions. This should not special-case a service or type name, and should retain the current protection against unresolved XML cref links in other documentation contexts.

Follow-up from #11288 and review comment #11288 (comment).

[github-actions]

🎯 Issue Triage

The C# client emitter's generated XML documentation for BinaryData union properties can reference types that are later removed by the provider reference map, resulting in docs that advertise inaccessible types. The fix requires a post-analysis mechanism to filter removed union-item types at write/render time rather than during description construction.

Category: Feature Request
Area: emitter:client:csharp

🔍 Analysis

• Affected component: PropertyDescriptionBuilder.CreateBinaryDataExtraDescription and GetUnionTypes in the C# emitter
• Root cause: Description building happens before ProviderReferenceMapAnalyzer has determined which types are removed, so removal checks at construction time are unreliable. The current workaround renders removed types as plain text instead of cref links, but the type is still mentioned in the docs.
• Desired behavior: Removed union-item types should be omitted entirely from generated BinaryData XML documentation, without special-casing specific service/type names.
• Related: Follow-up from Fix C# reference-map accessibility for hidden union types #11288 and this review comment.

📝 Suggested next steps

• Design a write-time or post-analysis hook that can filter removed types from BinaryData union documentation
• Ensure the existing guard against unresolved XML cref links is preserved in other documentation contexts
• Add a test covering the case where a removed type was previously advertised in BinaryData docs

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