fix(browser): resolve thread count from maxWorkers

[AriPerkkio]

Description

• Fixes Browser mode does not follow maxWorkers #7446

Allow users to define thread count in browser mode. This is useful when you don't want to allocate all CPUs to Vitest, see linked issue.

import { defineConfig } from "vitest/config";

export default defineConfig({
  test: {
    minWorkers: 4,
    maxWorkers: 4,

    browser: {
      enabled: true,
      headless: true,
      provider: "playwright",
      instances: [{ browser: "chromium" }],
    },
  },
});

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[AriPerkkio]

I'm not that familiar with Browser Mode's parallelism. Any reason why maxWorkers isn't used when resolving the value? It's already checking availableParallelism and os.cpu() so I think this makes sense.

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[sheremet-va]

I'm not that familiar with Browser Mode's parallelism. Any reason why maxWorkers isn't used when resolving the value? It's already checking availableParallelism and os.cpu() so I think this makes sense.

There is no reason, really. The issue is that parallelization is just not optimized at all, we still have the experimental tabs parallelization.