Keep-alive connection reuse stalls up to ~500ms on an idle event loop (possible regression in 8.8.0)

[hbacelar]

Bug Description

Since undici 8.8.0, reusing an idle keep-alive socket can stall for up to ~500ms before the
request is written to the wire. The request is created, queued, and then simply sits there.
This was discovered using a pool with a single connection.

I think cause is scheduleIdleSocketValidation() in lib/dispatcher/client-h1.js, which was changed
from an unref'd setTimeout(…, 0) to an unref'd setImmediate().

Used / Tested Environment

undici	8.9.0 (also reproduces on 8.8.0)
last good	8.7.0
Node.js	v24.18.0
Platform	macOS (darwin 25.5.0, arm64)

Reproduction

Standalone reproduction script:

import assert from 'node:assert'
import http from 'node:http'
import { test } from 'node:test'
import { Pool } from 'undici'

const REQUESTS = 6
const SERVER_DELAY_MS = 10
const MAX_TOTAL_MS = 400

test('reusing an idle keep-alive socket must not stall', async (t) => {
  const server = http.createServer((req, res) => {
    setTimeout(() => {
      res.writeHead(200, { 'content-length': 2 })
      res.end('ok')
    }, SERVER_DELAY_MS)
  })

  await new Promise((resolve) => server.listen(0, resolve))

  const pool = new Pool(`http://127.0.0.1:${server.address().port}`, { connections: 1 })

  t.after(async () => {
    await pool.close()
    server.close()
  })

  const durations = []
  const start = Date.now()

  for (let i = 0; i < REQUESTS; i++) {
    const requestStart = Date.now()
    const res = await pool.request({ path: '/', method: 'GET' })
    await res.body.text()
    durations.push(Date.now() - requestStart)
  }

  const total = Date.now() - start

  t.diagnostic(`per-request: ${durations.map((d) => `${d}ms`).join(', ')}`)
  t.diagnostic(`total: ${total}ms`)

  assert.ok(
    total < MAX_TOTAL_MS,
    `${REQUESTS} sequential requests took ${total}ms (per-request: ${durations.join('ms, ')}ms). ` +
      `The server replies in ${SERVER_DELAY_MS}ms, so this should be well under ${MAX_TOTAL_MS}ms.`
  )
})

Expected Behavior

reusing an idle keep-alive socket must not stall (84.42725ms)
per-request: 20ms, 13ms, 13ms, 12ms, 13ms, 13ms
total: 84ms

The server takes 10ms to respond, so every request should be ~10–20ms - and on 8.7.0 every one is.
On 8.9.0, reuses of the keep-alive socket cost hundreds of milliseconds each.

Actual Behavior

reusing an idle keep-alive socket must not stall (1482.400041ms)
per-request: 20ms, 458ms, 34ms, 51ms, 450ms, 464ms
total: 1477ms

  AssertionError [ERR_ASSERTION]: 6 sequential requests took 1477ms
  (per-request: 20ms, 458ms, 34ms, 51ms, 450ms, 464ms).

The server replies in 10ms, so this should be well under 400ms.