fix(postgres): inOpenTransaction must detect BEGIN without an assigned XID #787

Summary

PostgresConnection.inOpenTransaction probes pg_current_xact_id_if_assigned() IS NOT NULL (PG 13+). A BEGIN followed only by SELECT does not assign an XID, so the SQL-tab badge stays off and autocommit-off prepends a second BEGIN.

Scope

• Probe pg_stat_activity.xact_start for pg_backend_pid() or track BEGIN/COMMIT/ROLLBACK locally on the session.
• Autocommit-off must not emit BEGIN; concatenated with the next statement (see multi-statement execute issue).
• Cover BEGIN + SELECT 1 → tx open; COMMIT → closed.

Out of scope

• PG < 13 (document if the probe needs a fallback).