Fix intermittent crashes in Mac tests

[gonzaloriestra]

WHY are these changes introduced?

After #7029 the unit tests are failing sometimes with SIGABRT / ERR_PNPM_RECURSIVE_EXEC_FIRST_FAIL in MacOS.

Example: https://github.com/Shopify/cli/actions/runs/23306039213/job/67781905116

WHAT is this pull request doing?

• The forks → threads pool switch causes intermittent SIGABRT crashes on macOS CI runners. This is a known vitest issue where native modules (like @parcel/watcher, esbuild) aren't thread-safe and crash when loaded via worker_threads. The vitest docs recommend switching to forks pool as the fix.
• Use --pool forks for macOS CI jobs only; Linux and Windows keep threads for the ~12s speedup
• No impact on overall CI wall-clock time since macOS was never the bottleneck

How to test your changes?

CI

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	82.22%	14928/18157
🟡	Branches	74.65%	7393/9904
🟢	Functions	81.36%	3767/4630
🟢	Lines	82.61%	14115/17086

Test suite run success

3910 tests passing in 1502 suites.

Report generated by 🧪jest coverage report action from 2ee02ec

[gonzaloriestra]

Merge activity

• Mar 20, 2:07 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 20, 2:08 PM UTC: @gonzaloriestra added this pull request to the GitHub merge queue with Graphite.