workflow: a node prompt or step-result body cut at its bound keeps half a surrogate pair #2429

Written-by: Delivery-2

Two text bounds in plugins/workflow/src/graph.js cut with a bare String#slice, so a bound that lands between the halves of a surrogate pair stores a lone high surrogate — read as U+FFFD by any encoder and drawn as �.

Current vs expected

cleanLine sits 14 lines above cleanPrompt in the same file and already states the rule: "Cut on code points, never on UTF-16 units: a unit offset can split a surrogate pair and leave a lone surrogate, which renders as a replacement character." The same class was fixed for the host under #1993/#2000 and for the workflow node avatar under #2017; these two bounds were left behind.

The bound also counted UTF-16 units, so an emoji-heavy prompt reached only half the room an ASCII prompt has (a prompt of 4000 emoji was cut at 2000).

Reproduction (this base, 5e71a07e)

cd core && node -e "
import('../plugins/workflow/src/graph.js').then(m => {
  const out = m.cleanPrompt('a'.repeat(3999) + '🚀tail')
  console.log('prompt', out.length, out.codePointAt(out.length - 1).toString(16), out.isWellFormed())
  const r = m.readStepResult('x'.repeat(1999) + '🚀y', { outputs: [] })
  console.log('body  ', r.text.length, r.text.codePointAt(r.text.length - 1).toString(16), r.text.isWellFormed())
})"

prints prompt 4000 d83d false and body 2000 d83d false: both end in the lone high surrogate \ud83d.

Red first

plugins/workflow/test/prompt-bounds.test.js, written before the source change and run on this base with the source untouched — 3 fail / 0 pass:

not ok 1 - a prompt bound lands on a code point boundary, not inside an emoji
  expected: true
  actual: false
not ok 2 - the prompt bound counts code points, so whole emoji are never cut in half
  expected: 4000
  actual: 2000
not ok 3 - a step result body bound lands on a code point boundary, not inside an emoji
  expected: true
  actual: false

Scope

plugins/workflow/src/graph.js only: the shared code-point cut (cleanLine's existing behaviour, now one local helper), its use in cleanPrompt and readStepResult, and the new test. No maximum changes, no payload shape, no endpoint, no contract.