fix: abort the connection when a handler throws after the response started

[RaphaelFakhri]

Fixes an unhandled promise rejection when a handler throws after it has already started writing the response.

Problem

sendError() always calls send(), which sets Content-Length. If the handler already called res.write(), the headers are sent and setHeader() throws ERR_HTTP_HEADERS_SENT. The throw happens inside the catch of run(), so the promise from run() rejects with nothing to handle it. On current Node.js versions an unhandled rejection terminates the process.

module.exports = (req, res) => {
  res.write('partial');
  throw new Error('failed midway');
};

Changes

• sendError() checks res.headersSent. When the headers are already sent, it can't send an error response, so it destroys the connection unless the response has ended. The client sees a failed request instead of a truncated body that looks complete.
• Adds a test that throws after res.write() and asserts that run() resolves.

Testing

cd test && npx tap --ts suite/

Before the change, the new test fails with ERR_HTTP_HEADERS_SENT reported as an unhandled rejection. After the change, all three suites pass.