fix: observe rejection of a payload dropped after the reply was sent - #6936

Split out of #6526, as offered there — this half stands on its own and does not depend on whether send(promise) becomes a supported API.

The bug

send() drops the payload when the reply has already been sent:

if (this.sent === true) {
  this.log.warn({ err: new FST_ERR_REP_ALREADY_SENT(...) })
  return this
}

If that dropped payload is a thenable, nothing has attached a handler to it. A later rejection therefore escapes as an unhandled rejection, which terminates the process by default on modern Node.

This is reachable from plain JavaScript today regardless of the declared types — any second send() in a handler that has already replied:

fastify.get('/', function (req, reply) {
  reply.send('immediate')
  reply.send(somethingThatRejectsLater())   // dropped, rejection unobserved
})

The fix

Attach a handler to the dropped thenable that logs the rejection, so it is reported rather than fatal. The payload is still discarded and the existing FST_ERR_REP_ALREADY_SENT warning is unchanged.

Testing

Added a promise rejecting after the reply was sent is logged, not left unhandled, asserting the response is unaffected and the rejection reaches the logger.

• Fails on main (the rejection goes unhandled).
• Passes with the change: 83/83 in test/internals/reply.test.js, lint clean.

@mcollina — this is option (2) from my last comment on #6526. I've kept it deliberately narrow so it can be judged on its own; happy to close #6526 entirely if you'd prefer the feature not to land.