fix(postgres-js): widen PostgresJsSession constraint for TransactionSql

[YevheniiKotyrlo]

Summary

Widen PostgresJsSession's generic constraint from TSQL extends Sql to TSQL extends Sql | TransactionSql.

Problem

Since postgres@3.4.8, TransactionSql extends Omit<Sql, 'reserve'> instead of Sql. The transaction() method creates new PostgresJsSession<TransactionSql, ...>, but TransactionSql no longer satisfies TSQL extends Sql, producing TS2344.

This affects users regardless of skipLibCheck — any direct use of PostgresJsSession with the postgres package at v3.4.8+ triggers the error.

Fix

 export class PostgresJsSession<
-  TSQL extends Sql,
+  TSQL extends Sql | TransactionSql,

The union is backward-compatible: on older postgres versions where TransactionSql extends Sql, the | TransactionSql is redundant but harmless.

Related: #5187