Fix FromResponse generation for enum/extensible enum response types

[haiyuazhang]

When an API operation returns an extensible enum type (a readonly struct wrapping a string), the generator incorrectly emitted TypeName.FromResponse(result) which doesn't exist on enum structs. This fix checks IsEnum on the response type and generates 'new TypeName(result.Content.ToString())' instead.

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[Copilot]

Pull request overview

Fixes management generator output for operations whose response type is an enum/extensible-enum (readonly struct over string), where the generator previously emitted a non-existent TypeName.FromResponse(result) call.

Changes:

• Add an IsEnum check when generating Response.FromValue(...) for generic responses.
• For enum/extensible-enum response types, generate a direct construction of the response value instead of calling FromResponse.
• Keep existing T.FromResponse(result) generation for model response types.