fix: refcounting and error reporting in the libuv bindings

[algebraic-dev]

This PR fixes reference count, mark_mt and error messages in libuv modules.

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-08-14 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-08-15 02:18:37)

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ✅ Mathlib branch lean-pr-testing-14796 has successfully built against this PR. (2026-08-15 03:13:31) View Log

[TwoFX]

Here are two Codex-generated regression tests for a subset of the issues fixed in this PR if you want to include them:

module

import Std.Internal.UV.Signal

/-!
Tests that a signal handler can retry after libuv rejects starting it.
-/

open Std.Internal.UV

def startFails (signal : @& Signal) : IO Bool := do
  try
    discard signal.next
    return false
  catch _ =>
    return true

def test : IO (Bool × Bool) := do
  let signal ← Signal.mk 0 false
  let first ← startFails signal
  let second ← startFails signal
  signal.cancel
  return (first, second)

/-- info: (true, true) -/
#guard_msgs in
#eval test

module

import Std.Async.Basic
import Std.Internal.UV.TCP

/-!
Tests the error reported when a TCP shutdown is requested while one is already pending.
-/

open Std Async Net Internal.UV

def blockPromise (promise : IO.Promise (Except IO.Error α)) : IO α :=
  AsyncTask.ofPromise promise |>.block

partial def recvBytes (socket : TCP.Socket) (remaining : Nat) : IO Unit := do
  if remaining == 0 then
    return
  let some chunk ← blockPromise (← socket.recv? remaining.toUInt64)
    | throw <| IO.userError "unexpected end of stream"
  recvBytes socket (remaining - chunk.size)

def test : IO String := do
  let server ← TCP.Socket.new
  server.bind <| SocketAddressV4.mk (.ofParts 127 0 0 1) 0
  server.listen 1
  let addr ← server.getSockName

  let acceptPromise ← server.accept
  let client ← TCP.Socket.new
  let connectPromise ← client.connect addr
  blockPromise connectPromise
  let accepted ← blockPromise acceptPromise

  let size := 8 * 1024 * 1024
  let payload := ByteArray.mk <| Array.replicate size 0
  let sendPromise ← client.send #[payload]
  let shutdownPromise ← client.shutdown
  let message ← try
    discard client.shutdown
    pure "no error"
  catch
    | .otherError _ message => pure message
    | error => pure s!"unexpected error: {error}"

  recvBytes accepted size
  blockPromise sendPromise
  blockPromise shutdownPromise
  return message

/-- info: "shutdown already in progress" -/
#guard_msgs in
#eval test