[BUG][All Languages] Multi-Part content type in response ignores properties of composed schema (allOf/oneOf)

Created by: vvalchev

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

To define multipart content you typically do something like that:

   requestBody:
        content:
          multipart/form-data:
            schema:
              $ref: '#/components/schemas/ChangePasswordRequest'

However, if the schema is composed (using allOf) the inherited properties are ignored.

In the provided case, it generates a method without ANY parameters:

    void changeCurrentUserPassword();

openapi-generator version

4.3.0

OpenAPI declaration file content or url

https://gist.github.com/vvalchev/54a935c0f93c06a184a9eeb1640f96e6#file-api-yml

Command line used for generation

I'm using the following maven pom.xml file to generate the sources: https://gist.github.com/vvalchev/54a935c0f93c06a184a9eeb1640f96e6#file-pom-xml

Steps to reproduce

1. save pom.xml and api.yml
2. run mvn compile
3. check the generated sources

Related issues/PRs

Suggest a fix

The fix would be to handle correctly if the schema is composed. I've tried a simple patch:

diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index 82cebbf022..17edf00ddb 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -5517,8 +5517,11 @@ public class DefaultCodegen implements CodegenConfig {
         LOGGER.debug("debugging fromRequestBodyToFormParameters= " + body);
         Schema schema = ModelUtils.getSchemaFromRequestBody(body);
         schema = ModelUtils.getReferencedSchema(this.openAPI, schema);
-        if (schema.getProperties() != null && !schema.getProperties().isEmpty()) {
-            Map<String, Schema> properties = schema.getProperties();
+        List<String> allRequired = new ArrayList<String>();
+        Map<String, Schema> properties = new LinkedHashMap<>();
+        addProperties(properties, allRequired, schema);
+
+        if (!properties.isEmpty()) {
             for (Map.Entry<String, Schema> entry : properties.entrySet()) {
                 CodegenParameter codegenParameter = CodegenModelFactory.newInstance(CodegenModelType.PARAMETER);
                 // key => property name

After the patch, the generated code looks right:

// new code - after the patch has been applied
    void changeCurrentUserPassword(@FormParam(value = "password")  String password,@FormParam(value = "passwordConfirmation")  String passwordConfirmation,@FormParam(value = "oldPassword")  String oldPassword);

// old code
    void changeCurrentUserPassword();