test(migrations): make the 000096 idempotency subtest actually re-run the migration

The "backfill is idempotent" subtest called migrations.RunMigrations a
second time expecting it to re-exercise the DO block's guards. It does
not: m.Up() returns migrate.ErrNoChange once the database is already at
the latest version, so the migration body never runs again and the
subtest passed regardless of whether the guards worked. Proved by mutation:
stripping both idempotency devices from the up migration (the WHERE NOT
guard and the DISTINCT dedup on the backfill UPDATE) still left the old
test green.

Rewrites the subtest to read 000096_seed_ri_exchanger_group.up.sql and
execute its SQL directly a second time, the same pattern
000095_purchase_history_account_id_width_test.go already uses for its
re-run assertion. Re-verified by the same mutation: with the fix, stripping
the guards now fails the subtest (duplicate group_ids entry), and restoring
them passes it again.

The migration itself was already correctly idempotent; only the test
coverage was empty.

Author: cristim

internal/database/postgres/migrations/000096_seed_ri_exchanger_group_test.go

@@ -5,6 +5,8 @@ package migrations_test
 
 import (
 	"context"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/LeanerCloud/CUDly/internal/database/postgres/migrations"
@@ -116,8 +118,17 @@ func TestMigration_SeedRIExchangerGroup(t *testing.T) {
 		`, adminEmail, adminGroupIDForPurchaserTest, riExchangerGroupIDTest)
 		require.NoError(t, err)
 
-		// Re-running the seed must not duplicate the array entry.
-		require.NoError(t, migrations.RunMigrations(ctx, pool, migrationsPath, "", ""))
+		// Re-running migrations.RunMigrations here would NOT exercise the
+		// idempotency guards below: m.Up() returns ErrNoChange once the
+		// database is already at the latest version, so the migration body
+		// never runs a second time and this subtest would pass unconditionally
+		// regardless of whether the DO block's guards work. Reading the up
+		// migration file and executing its SQL directly re-applies the DO
+		// block for real, the same way 000095's re-run test does.
+		upSQL, err := os.ReadFile(filepath.Join(migrationsPath, "000096_seed_ri_exchanger_group.up.sql"))
+		require.NoError(t, err, "the up migration file must be readable")
+		_, err = pool.Exec(ctx, string(upSQL))
+		require.NoError(t, err, "re-running 000096 on an already-seeded database must be a no-op, not an error")
 
 		after := queryGroupIDsByEmail(t, ctx, pool, adminEmail)
 		count := 0