[AI assisted] MM-62755: Refactor scanning to map to a util (#30780)

With some neat generics, I was able to refactor
the scanning to a util function. I used it to
refactor 3 places and also removed an unnecessary method.

Claude was quite good here.

https://mattermost.atlassian.net/browse/MM-62755
```release-note
NONE
```

server/channels/store/sqlstore/utils.go

@@ -196,3 +196,27 @@ func quoteColumnName(driver string, columnName string) string {
 
 	return columnName
 }
+
+// scanRowsIntoMap scans SQL rows into a map, using a provided scanner function to extract key-value pairs
+func scanRowsIntoMap[K comparable, V any](rows *sql.Rows, scanner func(rows *sql.Rows) (K, V, error), defaults map[K]V) (map[K]V, error) {
+	results := make(map[K]V, len(defaults))
+
+	// Initialize with default values if provided
+	for k, v := range defaults {
+		results[k] = v
+	}
+
+	for rows.Next() {
+		key, value, err := scanner(rows)
+		if err != nil {
+			return nil, err
+		}
+		results[key] = value
+	}
+
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("error while iterating rows: %w", err)
+	}
+
+	return results, nil
+}