Fix OpenAPI tests failing on Postgres CI

openapiTestStores() was hardcoded to sqlite.NewStores() regardless of
the DB backend. When CI runs against Postgres, the SQLite ? placeholders
cause "syntax error at or near ','". Use the same IsSQLite() check
pattern as packages_bundled_test.go.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/handlers/openapi_test.go

@@ -7,7 +7,8 @@ import (
 
 	"switchboard-core/database"
 	"switchboard-core/store"
-	"switchboard-core/store/sqlite"
+	postgres "switchboard-core/store/postgres"
+	sqlite "switchboard-core/store/sqlite"
 )
 
 // Minimal static spec for testing — valid OpenAPI 3.0 structure.
@@ -32,7 +33,10 @@ func openapiTestStores(t *testing.T) store.Stores {
 	t.Helper()
 	database.RequireTestDB(t)
 	database.TruncateAll(t)
+	if database.IsSQLite() {
 		return sqlite.NewStores(database.TestDB)
+	}
+	return postgres.NewStores(database.TestDB)
 }
 
 // ── Test: Zero extensions → kernel spec only ──────────────────