Return ErrTxClosed from Rollback when the connection is already closed (#2557)

[luongs3]

Fixes #2557.

Problem

When a transaction query's context is canceled mid-flight, the low-level pgconn.PgConn is torn down (i/o timeout), but the outer pgx.dbTx state is left open (tx.closed stays false). The next Rollback then issues "rollback" on the already-dead connection and surfaces the raw "conn closed" error instead of pgx.ErrTxClosed.

This contradicts Rollback's documented contract:

Rollback will return an error where errors.Is(ErrTxClosed) is true if the Tx is already closed […]

A closed underlying connection means the transaction is, for all practical purposes, already closed — so callers reasonably expect ErrTxClosed (which they can assert on) rather than an opaque low-level error.

Reproduction (from the issue)

ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
defer cancel()

tx, _ := conn.BeginTx(ctx, pgx.TxOptions{})
_, _ = tx.Exec(ctx, "SELECT pg_sleep(1)") // canceled; pgconn is closed
err := tx.Rollback(context.Background())
// before: err == "conn closed"
// after:  errors.Is(err, pgx.ErrTxClosed) == true

Fix

In Rollback, check tx.conn.IsClosed() up front. If the connection is already closed, mark the tx closed and return ErrTxClosed. This mirrors how Commit already consults connection state (tx.conn.PgConn().TxStatus()), and leaves the normal rollback path — including the existing "rollback failed → die()" handling — untouched.

Tests

Added TestTransactionRollbackAfterCanceledQueryReturnsErrTxClosed, which reproduces the issue's exact scenario (cancel a pg_sleep query, then roll back) and asserts errors.Is(err, pgx.ErrTxClosed). It fails on master (conn closed) and passes with this change. The existing transaction test suite continues to pass.

[jackc]

As discussed in the issue, if any change is to be made, this isn't the direction we will go.