bug: when `serializer` in `httpLink` can not return `FormData`, even though tRPC can serialize it when using in `input` directly

  System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @tanstack/react-query: ^5.80.3 => 5.90.21 
    @trpc/client: canary => 11.9.1-canary.9+d92cc45b3 
    @trpc/next: canary => 11.9.1-canary.9+d92cc45b3 
    @trpc/react-query: canary => 11.9.1-canary.9+d92cc45b3 
    @trpc/server: canary => 11.9.1-canary.9+d92cc45b3 
    next: ^15.3.8 => 15.5.12 
    react: ^19.1.0 => 19.2.4 
    typescript: ^5.9.2 => 5.9.3 

import { parse, registerCustom, stringify } from "superjson";

let BLOBS: Blob[] = [];

registerCustom({
  isApplicable: (v) => v instanceof Blob,
  serialize: (blob) => {
    BLOBS.push(blob);
    return { index: BLOBS.length - 1 };
  },
  deserialize: (obj) => BLOBS[obj.index],
}, "Blob");

export function serialize(data: unknown): FormData | string {
  BLOBS = [];
  try {
    const serialized = stringify(data);
    if (BLOBS.length === 0) return serialized;
    const formData = new FormData();
    formData.append("data", serialized);
    for (const blob of BLOBS) formData.append("file", blob);
    console.log(formData);
    return formData;
  } finally {
    BLOBS = [];
  }
}

export function deserialize(val: FormData | string): unknown {
  let serialized: string;
  if (typeof val === "string") {
    serialized = val;
    BLOBS = [];
  } else {
    console.log(val);
    const data = val.get("data");
    if (typeof data !== "string") throw new Error("Expected 'data' field to be a string");
    const files = val.getAll("file");
    for (const file of files) {
      if (!(file instanceof Blob)) throw new Error("Expected 'file' field to be a Blob");
    }
    serialized = data;
    BLOBS = files as Blob[];
  }
  try {
    return parse(serialized);
  } finally {
    BLOBS = [];
  }
}