fix(test): restore alchemy/Test/Vitest harness

[sam-goodwin]

The internal migration to the alchemy-test runner (#797) deleted src/Test/Vitest.ts and dropped the vitest / @effect/vitest deps, unintentionally removing the public alchemy/Test/Vitest entrypoint that shipped through beta.63 (reported on Discord).

• Restore packages/alchemy/src/Test/Vitest.ts exactly as it was before the removal — every Test/Core.ts API it uses (toEffect, run, scratchStack, withProviders, deploy, destroy) is unchanged, so no porting was needed
• Re-add @effect/vitest to dependencies and vitest to devDependencies (both catalog:, same sections as before)
• Add an explicit ./Test/Vitest export entry matching ./Test/Bun / ./Test/Http (previously only reachable via the ./* wildcard)

Verified with bun tsc -b and a vitest smoke run exercising Test.make / beforeAll / test / test.skip / test.skipIf:

import * as Test from "alchemy/Test/Vitest";
import { expect } from "@effect/vitest";

const { test, beforeAll } = Test.make({ providers: ... });

The internal suite stays on alchemy-test; vitest.config.ts is intentionally not restored — consumers bring their own vitest config.

🤖 Generated with Claude Code

[alchemy-version-bot]

Install the packages built from this commit:

alchemy

bun add alchemy@https://pkg.ing/alchemy/f7102e2

@alchemy.run/better-auth

bun add @alchemy.run/better-auth@https://pkg.ing/@alchemy.run/better-auth/f7102e2

@alchemy.run/pr-package

bun add @alchemy.run/pr-package@https://pkg.ing/@alchemy.run/pr-package/f7102e2