test(ui): isolate toast bus state between tests

[DavidBabinec]

Summary

• reset the Toast bus singleton from global test teardown after React cleanup
• make Toast bus subscribers receive the current queue snapshot immediately, matching the documented contract
• add focused Toast bus coverage for snapshot delivery and test isolation

Why

The v0.0.6 release workflow failed in toastProvider.test.tsx because earlier module-inserter tests left success toasts in the global Toast bus. When the provider test published its own toasts, the snapshot included stale queue entries from previous tests.

Verification

• bun install
• bun test src/__tests__/ui/toastBus.test.ts src/__tests__/ui/toastProvider.test.tsx
• bun test src/__tests__/toolbar/modulePickerDropdown.test.tsx src/__tests__/ui/toastProvider.test.tsx
• bun test (5715 pass, 1 skipped bundle-size test before build, 0 fail)
• bun run build
• bun test src/__tests__/architecture/bundle-size-budgets.test.ts
• bun run lint
• git diff --check