Schema: PostgreSQL, paratest support and DataBuilder helpers #128

Part of #102. Line numbers refer to branch chore/php-8.4-dependabot (#98). Verified = reproduced, suspected = from reading the code.

• A PostgreSQL strategy (TRUNCATE … RESTART IDENTITY CASCADE, sequence reset). Today PostgreSQL fails with NoSchemaStrategyFound.
• Parallel-safe file and database names, e.g. a TEST_TOKEN suffix for paratest. Today parallel workers unlink each other's SQLite file and share one MySQL schema.
• DataBuilder helpers: return generated ids, default rows per table, references between rows, and assertTableContains() / assertRowCount().