Clear leftover begin() onclose handler after the transaction settles

[gepotumu]

Summary

• sql.begin() races the transaction against connection.onclose = reject so a disconnect during the transaction can fail the begin() promise.
• After a successful commit that handler is never cleared, so a later pool disconnect (idle timeout, sql.end(), server-side recycle) still calls reject. The raced promise is already settled, so this becomes unhandledRejection — often undefined.
• Clear connection.onclose in finally. Add a regression test that commits a transaction, then closes the pool and asserts no leftover rejection.

Test plan

• node tests/index.js (needs the usual postgres_js_test setup)
• Confirm the new test fails on current master and passes with this change
• After await sql.begin(...), close the connection via sql.end({ timeout: 0 }) or idle timeout and confirm no unhandledRejection

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