docs(eventcounter): clarify dedup comment in goal_count.sql

DISTINCT over the selected columns collapses exact duplicate rows; it is
not literally "one row per ID" if non-identical rows ever shared an ID.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: cre8ivejp

pkg/eventcounter/storage/v2/dwh_database/bigquery/sql/goal_count.sql

@@ -2,8 +2,9 @@ WITH deduped_events AS (
     -- BigQuery has no primary keys, so an at-least-once Pub/Sub redelivery
     -- can append the same event twice. Deduplicate by event ID before any
     -- aggregation, otherwise duplicates inflate event_count and value_sum.
-    -- Duplicates are identical rows, so DISTINCT over the used columns is
-    -- equivalent to picking one row per ID.
+    -- Duplicates are expected to be identical rows; DISTINCT collapses exact
+    -- duplicates for the columns used downstream without changing the
+    -- aggregation semantics.
     SELECT DISTINCT
         id,
         user_id,