fix(workspace): support overring pool and poolOptions on project level

[AriPerkkio]

Description

• Fixes Make it possible to override pool and poolOptions per workspace #4731

Adds support for specifying following options in workspace projects configurations:

• pool
• poolOptions.threads.isolate
• poolOptions.threads.singleThread
• poolOptions.vmThreads.isolate
• poolOptions.forks.isolate
• poolOptions.forks.singleFork

Example:

import { defineWorkspace } from 'vitest/config'

export default defineWorkspace([
  {
    // Tests with side effects - require isolation
    test: {
      name: 'DOM tests',
      include: ['./src/web/**'],
    },
  },
  {
    // Tests that don't have side effects - can be run without isolation
    test: {
      name: 'Side-effect free unit tests',
      include: ['./src/utils/**'],
      poolOptions: { 
        threads: {
          singleThread:  true,
          isolate: false
        }
      },
    },
  },
  {
    // Tests that require forks pool for compatibility reasons
    test: {
      name: 'Database tests',
      include: ['./src/db/**'],
      pool: 'forks',
    },
  },
])

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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