legacyprovision: fix RespError json serialization

[rnons]

A POC snippet

package main

import (
	"encoding/json"
	"log"
	"net/http"

	"go.mau.fi/mautrix-meta/pkg/connector"
)

func jsonResponse(w http.ResponseWriter, status int, response any) {
	w.Header().Add("Content-Type", "application/json")
	w.WriteHeader(status)
	_ = json.NewEncoder(w).Encode(response)
}

func main() {
	err := connector.ErrLoginChallenge
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		// Returns {"ErrCode": "...", "Err": "..."}
		jsonResponse(w, err.StatusCode, err)
		// Returns {"errcode": "...", "err": "..."}
		// err.Write(w)
	})

	log.Fatal(http.ListenAndServe(":8080", nil))
}

Seems RespError becomes any in jsonResponse, so func (e *RespError) MarshalJSON() ([]byte, error) is not used.

[tulir]

Becoming any doesn't prevent methods from being called, the actual issue is that bridgev2.RespError doesn't have MarshalJSON at all. The fix is good either way though