fix: fix partial index added diff (#5795)
[TML-2063](https://linear.app/prisma-company/issue/TML-2063/fix-another-partial-index-diffing-issue)
Fixes https://github.com/prisma/prisma/issues/29263
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## Summary by CodeRabbit
* **Bug Fixes**
* Fixed migration diff detection for partial indexes so manual partial
indexes are ignored when the preview feature is not enabled, preventing
false positives when comparing schemas and migrations.
* **Tests**
* Added tests to validate that manual partial indexes without the
preview feature do not produce migration differences.
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