Reject automatic filesystem mocks in tests

[dmerand]

WHY are these changes introduced?

Automatic filesystem mocks hide file and directory behavior. Filesystem tests must use real temporary files so they can detect incorrect I/O.

WHAT is this pull request doing?

Enable @shopify/cli/no-filesystem-auto-mocks as an error and replace the seven existing automatic filesystem mocks with temporary files or call-observation spies on real implementations. Dev-session tests also wait for event processing to finish before they check results.

The rule covers automatic mocks only. Factory and partial mocks remain outside its scope. It also adds a rule pointer to the filesystem section of the testing guide.

No changeset is needed: this changes internal lint tooling and tests, not CLI behavior.

How to manually test your changes?

In any test file, add vi.mock('@shopify/cli-kit/node/fs') and run ESLint on that file. The rule reports the automatic mock. A factory mock, such as vi.mock('@shopify/cli-kit/node/fs', async () => ({...})), is not reported.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add