chore: upgrade Vitest from 3.x to 4.x to fix flaky CI onTaskUpdate timeout

CI intermittently fails with:

```
Error: [vitest-worker]: Timeout calling "onTaskUpdate"
```

All tests pass but the unhandled error makes the run exit with code 1. This is a known Vitest 3.x bug where the birpc RPC channel between the main process and test worker has a hardcoded 60s timeout that fires independently of `testTimeout`. Under CI resource pressure, the worker blocks long enough to trigger it.

Fixed upstream in vitest-dev/vitest#8297 (merged Jul 2025), first available in `v4.0.0-beta.4`. Latest stable is `v4.0.18`.

**Upgrade scope:**
- `vitest`: `^3.2.4` → `^4.0.18`
- `@vitest/coverage-v8`: `^3.2.4` → `^4.0.18`
- `@effect/vitest`: `^0.25.1` → compatible version for vitest 4.x
- Verify all 971 unit tests pass
- Verify devnet tests pass
- Check for any breaking API changes in vitest 4.x