Resolves #7070(FR-2746)

## Summary

During FR-2609 + the BUI/root follow-up, 5 tests were skipped with `TODO(FR-2609)` markers because of two distinct compatibility issues. Both are now resolved.

### Group A — `@rc-component/motion` + jsdom 29 (4 tests)

- `packages/backend.ai-ui/src/components/BAIButton.test.tsx` (2 tests asserting the loading icon clears after async action completion)
- `packages/backend.ai-ui/src/components/BAIUnmountAfterClose.test.tsx` (2 tests asserting the Modal unmounts after the close animation finishes)

**Root cause**: antd v6 uses `@rc-component/motion` (CSSMotion) for Button loading-icon and Modal close transitions. The package probes vendor-prefixed style props on a `div` at module init to compute a `supportTransition` flag. Jest's older jsdom did not expose those props → flag resolved to `false` → motion completed synchronously. jsdom 29 (Vitest) exposes them → flag is `true` → motion waits for a `transitionend` event that jsdom never fires.

**Fix**: a MutationObserver in `setupTests.ts` watches the document for class additions matching `*-(leave|enter|appear)-active`, then queues a microtask to dispatch a synthetic `transitionend` on the element. This emulates what a real browser does when the CSS transition completes — rc-motion's listener fires, the active class is removed, and the assertion sees the expected post-transition DOM.

This was chosen after `vi.mock('@rc-component/motion/es/util/motion', ...)` failed to intercept the package-internal relative import (`./util/motion`). The MutationObserver works regardless of how `supportTransition` resolves and is the most robust fix.

### Group B — CJS `require('fs')` not interceptable by `vi.mock` (1 test)

- `scripts/i18n-merge-driver.test.ts` (`should parse JSON from file correctly`)

**Root cause**: the driver is a CommonJS `.js` file invoked by git via `node scripts/i18n-merge-driver.js %O %A %B` (per `.git/config`). It must remain CJS. Its inline `require('fs')` slips past Vitest's module-mock transform, so `vi.mock('fs', ...)` did not propagate.

**Fix**: rewrite the test to use a real temp file via `fs.mkdtempSync`, torn down in `afterEach`. This bypasses the CJS-mock incompat by not mocking, while still validating the observable behavior — `readJSON` parses JSON correctly and throws on invalid input.

**Bonus catch**: the existing `should throw error for invalid JSON` test was actually testing missing-file `ENOENT` (the mock had been silently failing in that test too). It now correctly tests invalid JSON parsing as labelled.

### CI tweak — root testTimeout + cjs coverage exclude

The first CI run on this PR exposed two coverage-related issues that didn't surface locally:

- `scripts/gen-theme-schema.test.ts > generates a valid JSON schema file` — exercises antd's type tree several times. Locally completes in ~2s; under V8 coverage instrumentation on CI's smaller runners it routinely exceeds the default 5 s per-test timeout.
- After that test failed, the davelosert action couldn't post a coverage comment because the `coverage-summary.json` was generated late / partially.

Two small adjustments to `vitest.config.ts` (root) make CI match local behaviour:
- `testTimeout: 30000` — generous per-test budget that matches what coverage adds in CI.
- `coverage.exclude: 'scripts/**/*.cjs'` — build tooling (the gen-theme-schema generator, the i18n merge driver) is exercised by the tests but its coverage % is not actionable; excluding it removes the heaviest source of v8 instrumentation overhead in this suite.

These are bundled here because the failing CI run on this PR was the first surfaceing of the issue; both adjustments are tightly scoped to root coverage runs.

## Verification

```
before:
  react/   856 pass | 0  skip
  BUI      315 pass | 5  skip
  root      90 pass | 1  skip
  total   1261 pass | 6  skip

after:
  react/   856 pass | 0  skip
  BUI      319 pass | 1  skip   ← 4 newly passing (Group A)
  root      91 pass | 0  skip   ← 1 newly passing (Group B)
  total   1266 pass | 1  skip
```

The remaining 1 skip in BUI is `BAIDomainSelect.test.tsx` `describe.skip` from FR-1731 (pre-existing, unrelated to this migration).

Local `pnpm exec vitest run --coverage` now produces `coverage/coverage-summary.json` for the root suite without timing out.

## Out of scope

- Fixing the FR-1731 `BAIDomainSelect` skip (separate concern).
- A global motion-disable opt-out beyond the helper — the MutationObserver is sufficient and minimally invasive.

## Stack

Top of the Vite migration stack (now 13 PRs). Builds on PR #7065 (`feat(FR-2744)` Vitest coverage reporting).