[WIP] Enable detectAsyncLeaks in Vitest tests

[Copilot]

• Add detectAsyncLeaks: true to packages/react/vitest.config.mts (node environment)
• Add detectAsyncLeaks: true to packages/doc-gen/vitest.config.mts (node environment)
• Add detectAsyncLeaks: true to packages/styled-react/vitest.config.ts (node environment)
• Add detectAsyncLeaks: true to packages/postcss-preset-primer/vitest.config.ts (node environment)
• Build validated successfully
• Code review: no issues
• CodeQL: no security alerts

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[changeset-bot]

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Latest commit: 79f49e2

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.