TCGC: Mark types only referenced through external alternate types with External usage in sdkPackage

[Copilot]

When a type has @@alternateType with an external type, child types reachable only through that external type (models, unions) still appeared in sdkPackage.models/sdkPackage.unions with full usage flags (Input, Output, Json, etc.). Emitters had no way to distinguish these from types they actually need to generate.

model Geometry { type: string; coordinates: numeric[]; }
model Feature {
  type: "Feature";
  geometry: Geometry | null;
  id?: string | numeric;
}
@@alternateType(Feature, { identity: "geojson::Feature", package: "geojson" }, "rust");

Previously: Geometry and the FeatureId union both appeared in the package with Input | Json usage. Now: they remain in sdkPackage.models/sdkPackage.unions but with only External usage, allowing emitters to distinguish them from types they need to generate.

Changes

• Block non-External usage on external types at the beginning of updateUsageOrAccess (types.ts): When encountering a type with external set, only UsageFlags.External is allowed to be set and propagated. All other usage (Input, Output, Json, etc.) and access propagation are blocked. The External flag itself still propagates to children so they are marked as reachable from external types.

• Updated tests (alternate-type.test.ts): Two existing tests updated to assert child types are present in getAllModels() with only External usage. New test added covering the exact Geometry/Feature/union scenario from the issue, verifying child types have External usage but no Input/Output usage.

[tadelesh]

@copilot Resolve the comments.

[Copilot]

@copilot Resolve the comments.

Both comments addressed in 9cefb6f. The external type check is now at the beginning of updateUsageOrAccess (before usage/access is set), and the filterOutTypes change has been removed.

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commit: 374f181

[azure-sdk]

All changed packages have been documented.

• ✅ @azure-tools/typespec-client-generator-core

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@azure-tools/typespec-client-generator-core - fix ✏️

Types that are only used within external alternate types are no longer included in sdkPackage models, enums, or unions.

[azure-sdk]

You can try these changes here

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