fix(pgsql): mark transaction state unknown when backend connection is broken

When a backend connection breaks with an error (e.g. postmaster SIGKILL
mid-query), libpq's PQtransactionStatus() returns PQTRANS_UNKNOWN rather
than PQTRANS_INTRANS, so IsActiveTransaction() reports false for a
connection that was in a transaction moments earlier. The retry path in
PgSQL_Session::handler_minus1_ClientLibraryError() then replays the
failed statement on a fresh backend connection as autocommit, breaking
transactional semantics: the client's next COMMIT lands on a connection
with no open transaction and PostgreSQL emits "WARNING: there is no
transaction in progress".

MySQL does not have this issue because mysql->server_status caches
SERVER_STATUS_IN_TRANS from the last OK packet and survives the
connection breaking.

Fix compute_unknown_transaction_status() to force
unknown_transaction_status=true when the backend is disconnected,
so the retry guard in handler_minus1_ClientLibraryError() refuses to
replay statements that were running inside an explicit transaction.

lib/PgSQL_Connection.cpp

@@ -1308,10 +1308,16 @@ void PgSQL_Connection::compute_unknown_transaction_status() {
 			return;
 		}
 
-		/*if (is_connected() == false) {
+		// On a broken backend, PQtransactionStatus() returns PQTRANS_UNKNOWN
+		// even if a transaction was active — libpq has no cached INTRANS bit
+		// equivalent to MySQL's server_status & SERVER_STATUS_IN_TRANS. Force
+		// unknown_transaction_status=true so IsActiveTransaction() still
+		// reports true and the retry path does not replay inside-tx statements
+		// on a fresh connection (which would run them as autocommit).
+		if (is_connected() == false) {
 			unknown_transaction_status = true;
 			return;
-		}*/
+		}
 
 		switch (PQtransactionStatus(pgsql_conn)) {
 		case PQTRANS_INTRANS: