Simplify calculation of error shapes in `EventStreamTestTools.kt` (#2130)

codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/testutil/EventStreamTestTools.kt

@@ -34,7 +34,6 @@ import software.amazon.smithy.rust.codegen.core.testutil.EventStreamUnmarshallTe
 import software.amazon.smithy.rust.codegen.core.util.hasTrait
 import software.amazon.smithy.rust.codegen.core.util.lookup
 import software.amazon.smithy.rust.codegen.core.util.outputShape
-import kotlin.streams.toList
 
 data class TestEventStreamProject(
     val model: Model,
@@ -115,16 +114,13 @@ object EventStreamTestTools {
         val project = TestWorkspace.testProject(symbolProvider)
         val operationSymbol = symbolProvider.toSymbol(operationShape)
         project.withModule(ErrorsModule) {
-            val errors = model.shapes()
-                .filter { shape -> shape.isStructureShape && shape.hasTrait<ErrorTrait>() }
-                .map { it.asStructureShape().get() }
-                .toList()
+            val errors = model.structureShapes.filter { shape -> shape.hasTrait<ErrorTrait>() }
             when (codegenTarget) {
                 CodegenTarget.CLIENT -> CombinedErrorGenerator(model, symbolProvider, operationSymbol, errors).render(this)
                 CodegenTarget.SERVER -> ServerCombinedErrorGenerator(model, symbolProvider, operationSymbol, errors).render(this)
             }
-            for (shape in model.shapes().filter { shape -> shape is StructureShape && shape.hasTrait<ErrorTrait>() }) {
-                StructureGenerator(model, symbolProvider, this, shape as StructureShape).render(codegenTarget)
+            for (shape in model.structureShapes.filter { shape -> shape.hasTrait<ErrorTrait>() }) {
+                StructureGenerator(model, symbolProvider, this, shape).render(codegenTarget)
                 requirements.renderBuilderForShape(this, codegenContext, shape)
             }
         }