fix(rust): maintain multi-line formatting for serde attributes in oneOf/anyOf enums

- Keep #[serde(untagged)] on separate line from pub enum for better readability
- Update test assertions to use two separate checks instead of linearize()
- Ensures generated Rust code maintains consistent formatting with existing samples
- Preserves the original multi-line attribute style preferred in Rust ecosystem

Author: timvw

modules/openapi-generator/src/main/resources/rust/model.mustache

@@ -126,7 +126,8 @@ impl Default for {{classname}} {
 {{#-first}}
 {{! Model with composedSchemas.oneOf - generate enum}}
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
-#[serde(untagged)] pub enum {{classname}} {
+#[serde(untagged)]
+pub enum {{classname}} {
 {{/-first}}
 {{/oneOf}}
 {{/composedSchemas}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/rust/RustClientCodegenTest.java

@@ -288,8 +288,8 @@ public void testAnyOfSupport() throws IOException {
         TestUtils.assertFileExists(modelIdentifierPath);
 
         // Should generate an untagged enum
-        String enumDeclaration = linearize("#[serde(untagged)] pub enum ModelIdentifier");
-        TestUtils.assertFileContains(modelIdentifierPath, enumDeclaration);
+        TestUtils.assertFileContains(modelIdentifierPath, "#[serde(untagged)]");
+        TestUtils.assertFileContains(modelIdentifierPath, "pub enum ModelIdentifier");
 
         // Should have String variant (for anyOf with string types)
         TestUtils.assertFileContains(modelIdentifierPath, "String(String)");