Follow-up to #281 (thanks for the fast fix — limitRequestBody preserving ServerRequest is exactly what we needed).

Problem

limitRequestBody / limitBodyStream always throw srvx's own ERR_BODY_TOO_LARGE (createBodyTooLargeError). A downstream framework with its own error type (h3's HTTPError) then has to re-recognize that foreign shape at every place a body is consumed:

• each body reader's catch (readBody/formData, validated json(), JSON-RPC) must special-case ERR_BODY_TOO_LARGE so it isn't masked as a 400 parse error, and
• the central error→response mapper must special-case it so a legit 413 isn't logged as an unhandled 500.

That's a code === "ERR_BODY_TOO_LARGE" check sprinkled across ~4 files (incl. the hot error path), purely to translate the error identity. See h3js/h3#1500.

Proposal

Let the caller inject the error thrown on overflow, defaulting to today's behavior:

export function limitRequestBody<T extends Request>(
  request: T,
  maxRequestBodySize: number,
  createError?: (maxRequestBodySize: number) => unknown, // default: createBodyTooLargeError
): T;

export function limitBodyStream(
  stream: ReadableStream<Uint8Array>,
  maxRequestBodySize: number,
  createError?: (maxRequestBodySize: number) => unknown,
): ReadableStream<Uint8Array>;

createError replaces the two internal createBodyTooLargeError(maxRequestBodySize) call-sites (the Content-Length fast-path and the limitBodyStream overflow). limitRequestBody forwards it to limitBodyStream and to the clone() branch so clones stay limited with the same error. Signature stays symmetric with createBodyTooLargeError(max); fully backward-compatible (omit → unchanged).

Then h3 passes its own HTTPError and drops all the translation code — no isBodyLimitError helper, no changes to the core error path, no bundle-size bump, and a raw req.text() overflow throws the framework's native error directly:

req = limitRequestBody(req, limit, () =>
  new HTTPError({ status: 413, statusText: "Request Entity Too Large",
    message: `Request body size exceeds the limit of ${limit} bytes` }));

Generally useful beyond h3 — any consumer can map overflow to its own error/response type without string-matching a code. Happy to send the PR if you're open to the shape.