validation
Optional SQL that validates a connection before handing it to a caller. If null, no validation is performed — the pool relies on SuspendConnectionPool.use to evict connections that raise errors during use. Typical values: "SELECT 1" (most databases), "SELECT 1 FROM DUAL" (Oracle).
Validation only runs when the candidate connection has been idle longer than validateAfterIdle — connections released within that window are assumed alive.