fix: GetDataStream must extend Writable, not legacy Stream

[bjarn]

fixes #18

Summary

MessageStream.get_data(cb) hangs forever on spooled messages when the constructor is passed a non-empty headers array. No 'error' event fires; the only symptom is that cb is never invoked, so any plugin reading the full body from data_post hits Haraka's plugin_timeout and DENYSOFTs the message.

Test coverage

Added one regression test: get_data() returns full body for spooled messages with ctor headers. It:

1. Creates a MessageStream with spool_after = 5 MB and a non-empty headers array (mimicking Haraka transactions).
2. Writes ~6 MB of body — large enough to spool and to fill internal highWaterMarks.
3. Calls get_data(cb) with a 3s timeout.
4. Asserts the callback fires with the full body.

Without this fix, the test times out after 3s with get_data callback never fired. With the fix it completes in ~70 ms.

Minimal repro

const MessageStream = require('haraka-message-stream')

const ms = new MessageStream(
  { main: { spool_after: 5 * 1024 * 1024, spool_dir: '/tmp' } },
  'repro',
  ['From: a@example.com\r\n'], // ctor headers is the trigger
)

ms.add_line('From: a@example.com\r\n')
ms.add_line('\r\n')
for (let i = 0; i < 100000; i++) ms.add_line('A'.repeat(76) + '\r\n')

ms.add_line_end(() => {
  console.log('spooling=', ms.spooling) // true
  const timeout = setTimeout(() => {
    console.log('❌ get_data never fired')
    process.exit(2)
  }, 5000)
  ms.get_data((buf) => {
    clearTimeout(timeout)
    console.log('✅ get_data fired, bytes=', buf.length)
    process.exit(0)
  })
})

Without the fix: prints ❌ get_data never fired after 5 s.
With the fix: prints ✅ get_data fired, bytes=7600000 after ~40 ms.

[msimerson]

Today I remembered why I didn't update to the new Writable yet: node v20 is still under LTS until 2026-04-18. Yep, 10 more days before it goes EOL. This PR surfaces a race condition in node v20. Fortunately, the CI tests caught it (🎉 for good tests!) and I'll have it fixed soon. Fun fun.