ci(ios-integration): run device tests group by group, not the whole file in one process

[vietnguyentuan2019]

The Integration (iOS Simulator) job has failed or timed out on every main push since at least May 2026 (continue-on-error: true, so it never blocked anything, but it also never gave useful signal). Verified on the reruns of the v1.8.2 merge commits: single-process run failed with no diagnostics; the code itself is fine — all 24 groups pass locally on an iOS simulator, ~17 min total.

Splits the run into one flutter test --plain-name "<group>" invocation per group() in the file, reports every failing group instead of stopping at the first, and raises the job timeout 40→75 min.

Verified on this branch (dispatched run 35830038193)

Completed in ~55 min (vs. hanging/failing on the old single-process run). 17/24 groups fully green. 7/24 have exactly one flaky test each, all the same shape — a 45-60s wait for a background task's completion event timing out, scattered across unrelated worker types (HTTP, image compression, constraints, concurrent tasks). That is CI-runner speed, not a regression: this job is still non-gating, and the failures are the pre-existing flakiness the file already documents.

Also confirms, for the first time in CI (not just locally): the #75 onStopped hook test passes on a real simulator run:

✅ Cancellation issue_75: cancelling a DartWorker fires its onStopped handler even when the callback never polls isTaskCancelled