GLOBAL_CONTEXT_KEY
The stand-in contextKey a global (unscoped) report carries.
A nullable contextId can't go into a MERGE key directly: Cypher property-map equality against a literal null matches nothing, including a node that has no such property, so a global report would take the CREATE branch on every retry and duplicate. contextKey is never null, so MERGE can key on it and a global report is as idempotent as a scoped one.