Fix failing postcss integration test

[thecrypticace]

This is what's causing the postcss test in #19434 to fail wanted to test this in isolation without the version bumps

[coderabbitai]

Walkthrough

Updated test code in the PostCSS integration suite: one test in integrations/postcss/core-as-postcss-plugin.test.ts now awaits the expectation (await expect(...)) instead of returning it, and two tests in integrations/postcss/index.test.ts were adjusted to match a different stderr message string (from does-not-exist is not exported from package to "./does-not-exist" is not exported). No other logic, error handling, public API, or exported entities were changed.

Pre-merge checks

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fix failing postcss integration test' clearly summarizes the main change - fixing a failing test in the postcss integration by adding missing await statements.
Description check	✅ Passed	The PR description directly relates to the changeset, referencing issue #19434 and explaining the purpose of adding missing await statements to tests.

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