Bulk-insert dedup key can collide

Problem/Motivation

$key = $target_id . $target_type . $source_id . $source_type . $source_langcode . ($source_vid ?: 0) . $method . $field_name;

Concatenating fields without a separator means two distinct tuples can theoretically produce the same string key (e.g. target_id=1 + target_type="2x" vs target_id=12 + target_type="x"), causing one row to silently overwrite another in $this->inserts before the bulk insert runs. Low likelihood given typical machine-name entity types, but cheap to fix by joining with a delimiter (e.g. "\0").

Proposed resolution

Use a delimiter.

Remaining tasks

User interface changes

API changes

Data model changes