#7899 test: add required Bun-only and outdated Node integration coverage

All Submissions:

• Have you followed the guidelines stated in CONTRIBUTING.md file?
• Have you checked to ensure there aren't any other open Pull Requests for the desired changed?

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

New Feature Submission:

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• Have you linted your code locally prior to submission?

Changes To Core Features:

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• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

Closes #7896

Add dedicated Linux, macOS, and Windows CI coverage for Bun-only frontend startup. Explicitly install Bun, verify startup without Node and with an outdated Node version, fail CI on missing prerequisites or skipped tests, and record runtime versions.