[Java] [resttemplate client] Array Path Parameters are not serialized according to Spec

Description

I have an OAS 3 spec with an operation that declares a path parameter of type array(string).
I have used the maven codegen plugin to generate a Java Client with the resttemplate library.

When invoking the service using the generated client, a failure may occur.
The root cause is that client does not control the serialization of the path argument:

// generated from resttemplate/api.mustache
public ResponseEntity<Void> myOperationWithHttpInfo(List<String> args) throws RestClientException {
   ...
   Map<String, Object> uriVariables = new HashMap();
      uriVariables.put("args", args);
      String path = UriComponentsBuilder.fromPath("/some/{args}/path").buildAndExpand(uriVariables).toUriString();

The UriComponentsBuilder#fromPath#buildAndExpand eventually just calls toString() on the List object, sending a URL
that depends on the implementation of List rather than the API specification.
For example, given List.of("a", "b", "c"), the client constructs a path "/some/[a,b,c]/path" instead of the expected "/some/a,b,c/path" (note the extra square brackets)

Swagger-codegen version

Swagger-codegen 2.4.31, with swagger-codegen-maven-plugin 3.0.42, on top of spring-web 5.3.19

Swagger declaration file content or url

openapi: 3.0.1
info:
  title: My API
  version: 0.0.0

paths:
  /some/{args}/path:
    get:
      operationId: myOperation
      parameters:
        - in: path
          name: args
          required: true
          schema:
            type: array
            items:
              type: string
      responses:
        200:
          description: "Internal error"
          content: { }

Command line used for generation

Swagger-codegen Maven Plugin version 3.0.42
Language: Java
Librar: resttemplate

Steps to reproduce

Generate the client as above
invoke client.myOperation(List.of("a","b","c"));
The operation may fail - possibly silently - depending on the implementation of toString() for the chosen List class

Related issues/PRs

Loosely related to issue #11987

Suggest a fix/enhancement

As a workaround, clients should make sure to use a List class that implements toString() as needed
For example, for a simple style, not exploded path argument:

class PathListAdapter<T> extends ArrayList<T> {
    // constructors and other overrides as needed
   public PathListAdapter(List<T> inner) {
      super(inner);
    }

    @Override
    public String toString() {
      return this.stream()
          .map(Objects::toString)
          .collect(Collectors.joining(","));
    }
}

This approach could be moved into the mustache template:

// generated from resttemplate/api.mustache
public ResponseEntity<Void> myOperationWithHttpInfo(List<String> args) throws RestClientException {
   ...
   Map<String, Object> uriVariables = new HashMap();
      uriVariables.put("args", new PathListAdapter<>(args));   // <---- wrap
      String path = UriComponentsBuilder.fromPath("/some/{args}/path").buildAndExpand(uriVariables).toUriString();

The choice of adapter (toString) should then be driven by the 'style' and 'expand' parameter attributes, for conformance with the spec.

I have looked briefly into subclassing the UriComponentsBuilder internal components, or trying to pass a formatter, but I have not been able to find an immediate, clean way that is not tightly coupled to the UriComponentsBuilder implementation.

Start with resttemplate/api.mustache and reproduce the generated client from the OpenAPI declaration in the issue, focusing on how the array path argument reaches UriComponentsBuilder. Done means generated clients serialize array path parameters according to their style and expand attributes rather than relying on the List implementation's toString().