[http-server-csharp] Emit C# class for models extending another model with no additional properties (#10175)

`isEmptyResponseModel` treats any model with zero own properties as
empty, skipping emission entirely. This means `model Baz extends Foo {}`
never produces a `Baz.cs` file, even though it's a meaningful derived
type users may want to extend via partial classes.

```typespec
model Foo { id: int32; name: string; }
model Baz extends Foo {}  // No Baz.cs emitted
```

### Changes

- **`packages/http-server-csharp/src/lib/utils.ts`**:
`isEmptyResponseModel` now checks `!model.baseModel` before treating
zero-property models as empty — derived models are always emitted.
- **`packages/http-server-csharp/test/generation.test.ts`**: Added test
case verifying a model extending another with no additional properties
produces a class file with correct inheritance.

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.chronus/changes/fix-empty-derived-model-emission-2026-3-27.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Emit C# class for models that extend another model with no additional properties

packages/http-server-csharp/src/lib/utils.ts

@@ -1059,7 +1059,7 @@ export function getControllerReturnStatement(
 
 export function isEmptyResponseModel(program: Program, model: Type): boolean {
   if (model.kind !== "Model") return false;
-  if (model.properties.size === 0) return true;
+  if (model.properties.size === 0 && !model.baseModel) return true;
 
   return (
     model.properties.size === 1 &&

packages/http-server-csharp/test/generation.test.ts

@@ -3395,3 +3395,30 @@ describe("collection type: defined as emitter option", () => {
     );
   });
 });
+
+it("emits class for model extending another model with no additional properties", async () => {
+  await compileAndValidateMultiple(
+    tester,
+    `
+      model Foo {
+        id: int32;
+        name: string;
+      }
+
+      model Baz extends Foo {}
+
+      @route("/foo/{id}") @get op getFoo(id: int32): Foo;
+      `,
+    [
+      [
+        "Foo.cs",
+        [
+          "public partial class Foo",
+          "public int Id { get; set; }",
+          "public string Name { get; set; }",
+        ],
+      ],
+      ["Baz.cs", ["public partial class Baz : Foo"]],
+    ],
+  );
+});