[FEATURE]: Read-only pending migrations check

### Feature hasn't been suggested before.

- [ ] I have verified this feature I'm about to request hasn't been suggested before. (It has: #2791 and discussion #5685, see Related. Left unticked on purpose.)

### Describe the enhancement you want to request

A read-only way to list pending migrations: the migrations present in the folder and absent from `drizzle.__drizzle_migrations`, without creating the table or applying anything.

Shape: an exported `getPendingMigrations({ migrationsFolder, db })` returning names, or a `dryRun: true` option on `migrate()`.

### Why

We run `migrate()` in a pre-deploy step and want the server to refuse to start while any migration is pending, so a deploy whose migrate step was skipped cannot serve on an old schema. Rails ships this as `check_all_pending!`.

Today we read the folder with `readMigrationFiles` and compare names against the table, mirroring what `getMigrationsToRun` does internally. That reimplements the migrator's selection rule and diverges silently the day the rule changes.

Matching by name rather than by a timestamp high-water mark matters, see #5769.

### Related

#2791 asks for this and has no maintainer reply since 2024. Discussion #5685 asks the same. Filed anew so it reaches the current queue, happy to have it closed into #2791 if that is where it should live.

Start by reading the existing readMigrationFiles and getMigrationsToRun paths, along with migrate and the drizzle.__drizzle_migrations table handling. Compare the requested API options with the prior requests in #2791 and discussion #5685; done means exposing a read-only pending-migration check that matches the migrator's name-based selection without creating tables or applying migrations.

Written by the indexing model from the issue text.