fix: support where argument on field-level @unique for partial indexes (#5774)
## 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
- [x] All existing partial index tests pass (52 total)
- [x] `make test-unit` passes
- [x] Verified with reproduction repo from prisma/prisma#29172. `prisma
generate` succeeds after fix
Closes prisma/prisma-engines#5773