fix(design/variants): surface real 240s timeout in AbortError message

[Pablosinyores]

Summary

generateVariant in design/src/variants.ts arms its abort timer at 240_000 ms:

const timeout = setTimeout(() => controller.abort(), 240_000);

but the AbortError branch returns "Timeout (120s)" — half the actual bound:

} catch (err: any) {
  clearTimeout(timeout);
  if (err.name === "AbortError") {
    return { path: outputPath, success: false, error: "Timeout (120s)" };
  }

A user staring at Timeout (120s) has no way to know whether to bump the orchestrator timeout, retry, or drop the call — the surfaced number is off by 2x from the timer that fired. This is purely a stale string literal, but it's the only signal a caller gets when the OpenAI Responses API stalls.

No linked issue — caught while auditing design/src/ after #1773 (the gpt-image-2 regression).

Fix

One-line literal correction in variants.ts:128: "Timeout (120s)" -> "Timeout (240s)".

Regression test

Adds a test to the existing design/test/variants-retry-after.test.ts that:

1. Stubs fetch to return a permanently-pending promise wired to the AbortSignal.
2. Monkey-patches setTimeout to fast-forward ONLY the 240_000 ms abort timer (delegating all other delays — including the retry-loop exponential backoff — to the real setTimeout so other behaviors aren't affected).
3. Awaits the resulting AbortError branch and asserts result.error === "Timeout (240s)".

Without the fix this test fails with the pre-fix "Timeout (120s)" literal.

Validation

$ bun test design/test/variants-retry-after.test.ts
 6 pass
 0 fail
 21 expect() calls
Ran 6 tests across 1 file. [8.05s]

(The existing 5 retry-after tests still pass; the new 6th test is the regression pin for the AbortError branch.)

AI assistance disclosure

Patch was drafted with AI assistance. Diff hand-reviewed against variants.ts:61 (the abort timer) and variants.ts:128 (the error branch); test exercises the actual AbortError code path rather than asserting the literal directly so a future timeout change keeps the message and the timer in lockstep.