fix: prevent crash when aborting a request with a non-stream body - #1911

Bug

Calling fetch() with a non-stream body (string, Buffer, Blob, URLSearchParams, etc.) and an already-aborted AbortController crashes the process:

const ac = new AbortController();
ac.abort();
fetch(url, { method: 'POST', body: 'hello world', signal: ac.signal });
// Unhandled 'error' event -> process terminates with exit code 1

The fetch promise rejects cleanly with AbortError, but the process still dies because the internally-created Stream.Readable.from(body) has no 'error' listener.

Root cause

In the Body constructor, the 'error' handler is attached only when the original body is a Stream:

if (body instanceof Stream) {
    body.on('error', ...);
}

For non-stream bodies, the actual stream used at runtime is Stream.Readable.from(...), which never gets a listener. When abort() calls request.body.destroy(error) on that un-listened stream, the error event is unhandled.

Fix

Attach the handler to stream (the value actually used) instead of body:

if (stream instanceof Stream) {
    stream.on('error', ...);
}

For stream bodies stream === body, so existing behavior is unchanged; for non-stream bodies the internally-created stream now gets the same handler.

Verification

• New regression test fails on unpatched code (crash), passes with the fix.
• Full AbortController describe block: 32 passing.
• Targeted test passes across all three AbortController variants (native, mysticatea, polyfill).