Uncatchable `AssertionError: assert(!this.paused)` on socket end

[andreypopp]

Description

If the consumer doesn't read the body immediately, the HTTP/1 parser pauses under
backpressure. When the socket ends (FIN), onHttpSocketEnd calls parser.finish(),
which asserts !this.paused. Since the parser is paused, this throws an uncatchable
AssertionError from the socket 'end' handler and crashes the process.

AssertionError [ERR_ASSERTION]: assert(!this.paused)
    at Parser.finish (.../undici/lib/dispatcher/client-h1.js:374:5)
    at Socket.onHttpSocketEnd (.../undici/lib/dispatcher/client-h1.js:951:30)

onHttpSocketEnd doesn't check parser.paused before calling finish().

Reproduction (minimised with Claude Code)

import { createServer } from "node:net";
import { fetch } from "undici";

const BODY = Buffer.alloc(64 * 1024, 0x61);

const server = createServer((sock) => {
  sock.once("data", () => {
    sock.write(
      "HTTP/1.1 200 OK\r\n" +
        `Content-Length: ${BODY.length}\r\n` +
        "Connection: close\r\n\r\n",
    );
    sock.write(BODY);
    sock.end(); // FIN
  });
});

server.listen(0, "127.0.0.1", async () => {
  const { port } = server.address();
  const res = await fetch(`http://127.0.0.1:${port}/`);
  void res; // never read the body -> parser pauses -> crash on FIN
  await new Promise((r) => setTimeout(r, 1000));
  console.log("no crash"); // never reached
});

The 64 KiB body is the body highWaterMark — the smallest size that pauses while still
arriving in one socket read, which makes this deterministic. It's not a fixed value:
the real trigger is "paused with the read buffer drained when FIN arrives," so the
size is network-dependent (intermittent across sizes over a real connection).

Expected vs actual

• Expected: clean end, or a catchable error. A peer closing a non-keep-alive
  response under backpressure shouldn't crash the process.
• Actual: uncatchable AssertionError, exit code 1.

Environment

• undici 8.3.0 (and current main)
• Node.js v22.22.2, Linux

[Azlond]

We started seeing this error in Node 24.17, and it still occurs in 24.18, which both use undici@7. Any chance the fix could get backported to the v7 release?
https://github.com/nodejs/undici/blame/v7.x/lib/dispatcher/client-h1.js#L367