Assert RI fast-path batches don't span subtransactions

A fast-path batch is filled and flushed within a single trigger-firing
cycle, so all rows in a batch come from the subtransaction that created
the cache entry.  AtEOSubXact_RI() relies on this: it identifies an
aborting subtransaction's entries by the subid stamped at entry
creation.  Assert the invariant in ri_FastPathBatchAdd() to catch any
future code that would violate it.

Suggested-by: Noah Misch <noah@leadboat.com>
Discussion: https://postgr.es/m/20260705222115.be.noahmisch@microsoft.com
Backpatch-through: 19

Author: amitlan

src/backend/utils/adt/ri_triggers.c

@@ -2907,6 +2907,14 @@ ri_FastPathBatchAdd(RI_ConstraintInfo *riinfo,
 		return;
 	}
 
+	/*
+	 * A batch is filled and flushed within a single trigger-firing cycle, so
+	 * every row added to an entry comes from the subtransaction that created
+	 * it.  AtEOSubXact_RI() relies on this to identify an aborting
+	 * subtransaction's entries by the subid stamped at entry creation.
+	 */
+	Assert(fpentry->subid == GetCurrentSubTransactionId());
+
 	/*
 	 * Buffer the row.  A full batch is flushed below and re-entry is handled
 	 * above, so there is always room here; the bounds check just guards the