fix: remove vestigial spy stub, import directly from @vitest/spy

[mrginglymus]

Description

• Fixes Built .d.ts files contain js code #7571

The integration/spy.ts file has been vestigial since #2575. It seems to be confusing rollup-dts-plugin. Rather than bang our heads against a wall fixing that, remove it and just import/export from @vitest/spy directly where referenced.

This appears to fix the output .d.ts files.

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[mrginglymus]

Ahh...not as vestigial as I'd hoped!