chore: upgrade to vitest v4 (#18265)

## Summary

Bumps `vitest` and `@vitest/coverage-v8` from `^2.1.9` to `^4.1.7` (two
major versions). Three small test-harness updates compensate for vitest
4 behavior changes; all 7569 tests still pass.

- **`packages/svelte/tests/runtime-browser/test.ts`** — vitest 4 removed
the deprecated `describe(name, fn, opts)` signature. Pass options as the
second argument.
- **`packages/svelte/tests/runtime-legacy/shared.ts`** — vitest 4's
jsdom env binds `virtualConsole` to the original `globalThis.console`
reference *before* vitest wraps the console, so inline-`<script>` logs
and `jsdomError` events no longer reach per-test `console.{log,error}`
overrides. Restore vitest 2's behavior by re-routing the virtual console
through the live `console` in `beforeAll`. Also promote the window-error
listener to a named function and remove it in `finally` — previously
leaked listeners from earlier tests kept writing to module-level
`unhandled_rejection`, polluting later tests.
- **`vitest.config.js`** — bump `testTimeout` to 10s. The 5s default
trips a handful of dev-mode tests that exercise
`effect_update_depth_exceeded`, whose ~1000 Error-stack captures per
flush are slower under vitest 4's deeper async stacks.

## Test plan

- [x] `pnpm test` — 7569 passed, 63 skipped, 0 failed

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>