Simplify test model names

- Rename modelWithEmptyStruct to inputUnionWithEmptyStructure
- Remove output parameter and TestOutput structure
- Focus tests on input serialization only

Author: vcjana

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/protocols/AwsQueryTest.kt

@@ -34,7 +34,7 @@ class AwsQueryTest {
         }
         """.asSmithyModel()
 
-    private val modelWithEmptyStruct =
+    private val inputUnionWithEmptyStructure =
         """
         namespace test
         use aws.protocols#awsQuery
@@ -47,18 +47,13 @@ class AwsQueryTest {
         }
 
         operation TestOp {
-            input: TestInput,
-            output: TestOutput
+            input: TestInput
         }
 
         structure TestInput {
             testUnion: TestUnion
         }
 
-        structure TestOutput {
-            testUnion: TestUnion
-        }
-
         union TestUnion {
             // Empty struct - should generate _inner to avoid unused variable warning
             emptyStruct: EmptyStruct,
@@ -83,6 +78,6 @@ class AwsQueryTest {
     fun `union with empty struct generates warning-free code`() {
         // This test will fail with unused variable warnings if the fix is not applied
         // clientIntegrationTest enforces -D warnings via codegenIntegrationTest
-        clientIntegrationTest(modelWithEmptyStruct) { _, _ -> }
+        clientIntegrationTest(inputUnionWithEmptyStructure) { _, _ -> }
     }
 }

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/protocols/RestJsonTest.kt

@@ -39,7 +39,7 @@ internal class RestJsonTest {
         }
         """.asSmithyModel()
 
-    private val modelWithEmptyStruct =
+    private val inputUnionWithEmptyStructure =
         """
         namespace test
         use aws.protocols#restJson1
@@ -54,18 +54,13 @@ internal class RestJsonTest {
 
         @http(uri: "/test", method: "POST")
         operation TestOp {
-            input: TestInput,
-            output: TestOutput
+            input: TestInput
         }
 
         structure TestInput {
             testUnion: TestUnion
         }
 
-        structure TestOutput {
-            testUnion: TestUnion
-        }
-
         union TestUnion {
             // Empty struct - RestJson ALWAYS uses inner variable, no warning
             emptyStruct: EmptyStruct,
@@ -92,6 +87,6 @@ internal class RestJsonTest {
         // Unlike RestXml/AwsQuery, RestJson serializers always reference the inner variable
         // even for empty structs, so no underscore prefix is needed.
         // This test passes without any code changes, proving RestJson immunity.
-        clientIntegrationTest(modelWithEmptyStruct) { _, _ -> }
+        clientIntegrationTest(inputUnionWithEmptyStructure) { _, _ -> }
     }
 }

codegen-client/src/test/kotlin/software/amazon/smithy/rust/codegen/client/smithy/protocols/RestXmlTest.kt

@@ -83,7 +83,7 @@ internal class RestXmlTest {
 
         """.asSmithyModel()
 
-    private val modelWithEmptyStruct =
+    private val inputUnionWithEmptyStructure =
         """
         namespace test
         use aws.protocols#restXml
@@ -98,18 +98,13 @@ internal class RestXmlTest {
 
         @http(uri: "/test", method: "POST")
         operation TestOp {
-            input: TestInput,
-            output: TestOutput
+            input: TestInput
         }
 
         structure TestInput {
             testUnion: TestUnion
         }
 
-        structure TestOutput {
-            testUnion: TestUnion
-        }
-
         union TestUnion {
             // Empty struct - should generate _inner to avoid unused variable warning
             emptyStruct: EmptyStruct,
@@ -133,6 +128,6 @@ internal class RestXmlTest {
     fun `union with empty struct generates warning-free code`() {
         // This test will fail with unused variable warnings if the fix is not applied
         // clientIntegrationTest enforces -D warnings via codegenIntegrationTest
-        clientIntegrationTest(modelWithEmptyStruct) { _, _ -> }
+        clientIntegrationTest(inputUnionWithEmptyStructure) { _, _ -> }
     }
 }