Stop tests leaking UserDefaults suite domains

[robinebers]

TL;DR

The test suite created unique UserDefaults(suiteName:) domains but usually cleared them only before a test, never afterward, so repeated swift test runs accumulated thousands of test preference plists in ~/Library/Preferences. This routes every leaking scratch suite through one shared helper that removes its data in teardown. Test infrastructure only — no app behavior change.

What was happening

• Test helpers built suites with fresh UUIDs, called removePersistentDomain once at the top for a clean slate, and returned them.
• Nothing removed those domains when the test finished, so each run left newly written preferences behind.
• A few files already used a correct defer { removePersistentDomain } pattern; those remain unchanged.

What this changes

• Adds XCTestCase.makeScratchDefaults(suiteName:) in TestSupport.swift. It pre-clears the suite and registers an addTeardownBlock that removes its persistent domain after the test.
• Routes all 28 leaking call sites across 27 test files through that helper, including the newer LayoutBootstrapTests and LayoutPersistenceTests added after this branch was first created.
• Keeps the existing XCTest class hierarchy unchanged and continues returning plain UserDefaults instances to test bodies.

Heads-up

• macOS 27 caveat: on macOS ≤ 26 (including the macos-26 CI runner), removePersistentDomain removes the suite plist. On macOS 27 beta, cfprefsd recreates an empty 42-byte dictionary for touched suites when the test process exits. The teardown still removes all test data, but a post-run filesystem cleanup is required to remove those empty shells.
• A complete one-time cleanup must include the non-OpenUsageTests prefixes used by Copilot, notification, and snapshot-cache tests:

find "$HOME/Library/Preferences" -maxdepth 1 -type f \( \
  -name 'OpenUsageTests*.plist' -o \
  -name 'CopilotProviderTests-*.plist' -o \
  -name 'WidgetDataStoreNotificationTests*.plist' -o \
  -name 'providerSnapshotCache.test.*.plist' \
\) -delete

Tests

• swift test --filter 'Layout(Bootstrap|Persistence)Tests' — 5 passed.
• swift test — 953 passed, 3 skipped, 0 failures.
• Verified from a purged clean slate on macOS 27: the full suite created 216 plist shells, all 42-byte empty dictionaries; zero matching files retained test data.
• Removed the pre-existing 5,623-file local backlog and the post-verification empty shells; both matching filesystem files and defaults domains were zero afterward.

[robinebers]

I found one issue that should be fixed before merging.

This covered every leaking test when the branch was created, but two layout test groups were added to main afterward. Their temporary preferences still get cleared only before the test, not after it. That means a full test run would continue leaving those two sets of saved data behind, while the PR says it stops the leak everywhere.

Please update the branch from main and route both newer layout helpers through the shared cleanup helper as well. The shared helper and the rest of this cleanup look sound.