fix: support where argument on field-level @unique for partial indexes

[jay-l-e-e]

fix: support where argument on field-level @unique for partial indexes

Summary

• Add where clause parsing to field-level @unique, fixing "No such argument" validation error when db pull generates @unique(where: raw("...")) for single-column partial unique indexes.

Changes

• psl/parser-database/src/attributes.rs: Call parse_where_clause() in visit_field_unique, matching the existing behavior in model_unique / model_index.
• psl/psl/tests/attributes/partial_index.rs: 4 new tests covering raw syntax, object syntax, missing preview feature error, and map + where combination.

Test plan

• All existing partial index tests pass (52 total)
• make test-unit passes
• Verified with reproduction repo from prisma generate schema validation throws No such argument error for partial / filtered field level indexes  prisma#29172. prisma generate succeeds after fix

Closes #5773

[jay-l-e-e]

@jacek-prisma Please review this PR. Thank you!

[codspeed-hq]

Merging this PR will not alter performance

✅ 11 untouched benchmarks
⏩ 11 skipped benchmarks¹

---

_{Comparing jay-l-e-e:fix/field-level-unique-where-clause (c06c2ce) with main (ab56fe7)}

Footnotes

1. 11 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩