fix(db-postgres): keep generated down migrations idempotent

[youcefzemmar]

What

Auto-generated migrations on Postgres can fail on the down step (and sometimes on the up step when a collection is dropped) because drizzle-kit emits explicit DROP CONSTRAINT / DROP INDEX statements that follow a DROP TABLE … CASCADE. By the time those explicit drops run, Postgres has already removed the dependent objects, so each subsequent drop errors out with constraint … does not exist / index … does not exist.

This PR rewrites the generated statements so each DROP CONSTRAINT and DROP INDEX carries IF EXISTS. The rewrite happens once, at migration-generation time inside sanitizeStatements, so existing user migrations are not touched — only newly created files.

Why

DROP TABLE "x" CASCADE; followed by ALTER TABLE "x_rels" DROP CONSTRAINT "x_rels_x_fk"; is a common shape in down migrations that delete a collection. The CASCADE has already removed x_rels_x_fk as a side effect, so the explicit drop fails and the whole migration aborts mid-way. IF EXISTS lets drizzle-kit's ordering survive Postgres's dependency-resolution behavior without changing the generator upstream.

The same pattern already lives in the v2→v3 predefined migration (groupUpSQLStatements.ts:156), which converts DROP CONSTRAINT to DROP CONSTRAINT IF EXISTS for the same reason. This PR generalizes that fix to every Postgres migration the adapter writes.

Fix

• New utility packages/drizzle/src/utilities/makePostgresDropsIdempotent.ts — pure transform over string[] of generated SQL. Anchored on \bDROP CONSTRAINT\b / \bDROP INDEX\b with a negative lookahead for IF EXISTS, so it's safe to run more than once and won't touch CREATE INDEX or quoted strings that don't contain those tokens.
• Re-exported via @payloadcms/drizzle/postgres.
• Wired into sanitizeStatements in packages/db-postgres/src/index.ts and packages/db-vercel-postgres/src/index.ts. SQLite adapters are unaffected — SQLite doesn't support ALTER TABLE … DROP CONSTRAINT and doesn't exhibit the regression.

Testing

• Added packages/drizzle/src/utilities/makePostgresDropsIdempotent.spec.ts covering: single-statement rewrites for both DROP CONSTRAINT and DROP INDEX, idempotency when IF EXISTS is already present, no-ops on CREATE INDEX / DROP TABLE / ADD COLUMN, and multi-drop ALTER TABLE statements.
• Verified the regex behavior directly with node against the same five cases — all pass.
• pnpm install is broken in my local sandbox (unable to open database file from pnpm's content-addressable store), so I couldn't run the project's vitest/eslint locally; the spec runs in CI under the unit project (vitest.config.ts:52).

Notes

• The existing test/database/up-down-migration/int.spec.ts is gated on PAYLOAD_DATABASE=postgres and won't exercise this branch in default CI. Left untouched on purpose — the unit test of the transform is the load-bearing test for this fix, and the int spec would need a multi-config setup (create collection, migrate up, drop collection, migrate up/down) to reproduce end-to-end. Happy to follow up on that scenario if you'd like it in this PR.
• The transformation runs at generation time, so user-owned existing migration files in repos are not rewritten — consistent with how Payload treats those files as authored artifacts.

Fixes #14800.