Improve behaviour of idle_in_transaction_session_timeout

[davibe]

Related to #1133

When PostgreSQL kills a connection due to idle_in_transaction_session_timeout, two issues could occur:

1. Uncatchable crash: nextWrite() attempted to write to a null socket
2. Silent execution outside transaction!!: If pool had pending queries, the connection would reconnect and subsequent txSql queries could execute on the new connection—outside of the intended transaction context—causing potential data corruption

Changes:

src/connection.js:

• Set terminated = true in closed() before nulling socket
• Add null guard in nextWrite() to prevent crash

src/index.js:

• Track closed state in begin() scope
• Reject txSql queries immediately if connection was closed, preventing execution after reconnect

This ensures txSql always fails with CONNECTION_CLOSED when the underlying connection is killed, regardless of whether the connection object later reconnects for other pool queries.