Client form bodies ignore encoding style/explode, so arrays are always sent as name[0]=… #2604

Summary

For application/x-www-form-urlencoded request bodies, the generated client encodes array properties as indexed keys (topic[0]=news&topic[1]=releases), even when the spec declares style: form, explode: true for the property. Per the OpenAPI 3.0 Encoding Object, that serializes as repeated keys: topic=news&topic=releases. A server that reads repeated keys sees no topic at all.

This looks like the same problem as #1575, which was closed as a JSON body question; the body there was form-encoded too.

Reproduction

openapi.yaml:

openapi: 3.0.3
info:
  title: repro
  version: 1.0.0
paths:
  /subscriptions:
    post:
      operationId: subscribe
      requestBody:
        required: true
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              required: [email, topic]
              properties:
                email:
                  type: string
                topic:
                  type: array
                  items:
                    type: string
            encoding:
              topic:
                style: form
                explode: true
      responses:
        '204':
          description: Subscribed

cfg.yaml:

package: api
generate:
  models: true
  client: true
output: api/api.gen.go

main.go:

package main

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"net/http/httptest"

	"repro/api"
)

func main() {
	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		body, _ := io.ReadAll(r.Body)
		fmt.Printf("body: %s\n", body)
		w.WriteHeader(http.StatusNoContent)
	}))
	defer server.Close()

	client, _ := api.NewClient(server.URL)
	_, _ = client.SubscribeWithFormdataBody(context.Background(), api.SubscribeFormdataRequestBody{
		Email: "jane@example.com",
		Topic: []string{"news", "releases"},
	})
}

$ go mod init repro
$ go run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen@v2.8.0 -config cfg.yaml openapi.yaml
$ go mod tidy
$ go run .
body: email=jane%40example.com&topic%5B0%5D=news&topic%5B1%5D=releases

Expected body: email=jane%40example.com&topic=news&topic=releases.

Cause

• The client template always calls runtime.MarshalForm(body, nil) (pkg/codegen/templates/client.tmpl), so the spec's encoding never reaches the runtime.
• runtime.MarshalForm only reads RequestBodyEncoding.ContentType. Style and Explode are unused, and marshalFormImpl always writes slice elements as name[i].

runtime.BindForm accepts repeated keys, name[] and name[i], so a generated client works against a generated server either way. The mismatch only shows up against other servers.

Proposal

Honour style/explode when the spec declares them explicitly: the generator passes the operation's encoding to MarshalForm instead of nil, and MarshalForm writes repeated keys for arrays with style: form, explode: true. Bodies without an explicit encoding keep today's output, so the change isn't breaking.

I can open PRs for both oapi-codegen and runtime if this direction sounds right.

oapi-codegen v2.8.0, runtime v1.7.0, Go 1.26.8