test: fix Node 26 flake in 'group parts with same name to an array'

[mcollina]

Summary

Fixes a deterministic test failure on Node 26 (CI Test (26, ubuntu-latest)):

test at test/multipart.test.js:127:1
✖ should group parts with the same name to an array
  'plan expected 15 assertions but received 16'

This is unrelated to any behaviour change; it is a test-only flake. The docs PR (#645) shows the same failure with no runtime changes.

Root cause

The test used t.plan(15) and asserted the number of times the streaming loop observed a grouped array via conditional t.assert.ok() calls:

if (Array.isArray(part.fields.upload)) {
  t.assert.ok('multiple fields are grouped by array')
}
if (Array.isArray(part.fields.hello)) {
  t.assert.ok('multiple files are grouped by array')
}

How many times these fire depends on when busboy turns a repeated field name into an array, which is timing-sensitive. Under Node 26 the timing shifts and one extra assertion fires, so the plan of 15 is exceeded (16 received).

Fix

Track whether the grouping was observed in flags during the loop, then assert the result once and deterministically after the loop:

let seenUploadArray = false
let seenHelloArray = false
for await (const part of parts) {
  t.assert.ok(part)
  if (Array.isArray(part.fields.upload)) seenUploadArray = true
  if (Array.isArray(part.fields.hello)) seenHelloArray = true
  if (part.file) await streamToNull(part.file)
}
t.assert.ok(seenUploadArray)
t.assert.ok(seenHelloArray)

The per-part t.assert.ok(part) count (6 parts) and the two response-handler assertions are unchanged; the plan is now 10 and no longer depends on streaming timing.

Verification

• node --test test/multipart.test.js — 13/13 pass
• npm run test:unit — 92/92 pass, 100% coverage maintained
• npm run lint — clean