[dart][dart-dio] Support OpenAPI 3.1 composed schemas (#20475)

Author: jutuon

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractDartCodegen.java

@@ -2,7 +2,6 @@
 
 import com.google.common.collect.Sets;
 import io.swagger.v3.oas.models.Operation;
-import io.swagger.v3.oas.models.media.ComposedSchema;
 import io.swagger.v3.oas.models.media.Schema;
 import io.swagger.v3.oas.models.media.StringSchema;
 import io.swagger.v3.oas.models.servers.Server;
@@ -593,7 +592,7 @@ public CodegenProperty fromProperty(String name, Schema p, boolean required) {
 
         // Handle composed properties and it's NOT allOf with a single ref only
         if (ModelUtils.isComposedSchema(p) && !(ModelUtils.isAllOf(p) && p.getAllOf().size() == 1)) {
-            ComposedSchema composed = (ComposedSchema) p;
+            Schema<Object> composed = (Schema<Object>) p;
 
             // Count the occurrences of allOf/anyOf/oneOf with exactly one child element
             long count = Stream.of(composed.getAllOf(), composed.getAnyOf(), composed.getOneOf())