[http-client-csharp] Preserve enum type when @alternateType collapses single-member enum to constant

[Copilot]

When @alternateType("green") is applied to a required property of type Colors (a single-member enum), TCGC resolves it as an SdkConstantType instead of SdkEnumType. The emitter's case "constant": branch only promoted constants to extensible enums for optional/nullable properties, so required properties collapsed to string literals.

Changes

• emitter/src/lib/type-converter.ts: In the case "constant": branch, check sdkProperty.__raw?.type?.kind === "Enum" to detect when the original TypeSpec property type was an enum. When true, convert to extensible enum regardless of optionality.
• emitter/test/Unit/constant-type.test.ts: Added test for single-member enum with @alternateType on a required property, asserting kind === "enum" and isFixed === false.

Repro

enum Colors { green }

model EmployeeProperties {
  @Azure.ClientGenerator.Core.alternateType("green")
  favoriteColor: Colors;
}

Before: favoriteColor emitted as constant "green"
After: favoriteColor emitted as extensible enum type

[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/http-client-csharp@10281

commit: 7b71584

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[jorgerangel-msft]

Is there ever a case where we would want prop to be a constant and not an extensible enum? Also, ideally, we would validate this doesn't cause any change in behavior for existing libraries. I am not aware of any that do this, but there might be. @JoshLove-msft - any thoughts on this behavioral change?

[ArcturusZhang]

enum Colors { green }

model EmployeeProperties {
@Azure.ClientGenerator.Core.alternateType("green")
favoriteColor: Colors;
}
Before: favoriteColor emitted as constant "green"
After: favoriteColor emitted as extensible enum type

To me, this does not really make sense.
This alternateType should work as if we wrote this in typespec:

model EmployeeProperties {
  favoriteColor: "green";
}

if we really want extensible enum, we should do this:

enum Colors { green }
union ReplacedColors { ...Colors, string }

model EmployeeProperties {
  @Azure.ClientGenerator.Core.alternateType(ReplacedColors)
  favoriteColor: Colors;
}

FYI - TCGC emits fixed enum for "closed unions" like "a" | "b", it only emits extensible enum when there is a string in the union like "a" | "b" | string.