Update ppl-bugfix-reference: prefer datarows match over length check

YAML REST tests should assert actual row content, not just count.
Learned from #5174 where total-only assertion missed incorrect values.

Signed-off-by: Heng Qian <qianheng@amazon.com>

Author: qianheng-aws

.claude/harness/ppl-bugfix-reference.md

@@ -109,9 +109,13 @@ teardown:
       headers: { Content-Type: 'application/json' }
       ppl: { body: { query: "source=test_issue_<ISSUE> | <your PPL>" } }
   - match: { total: <expected> }
-  - length: { datarows: <expected> }
+  - match: { datarows: [ [ <row1_val1>, <row1_val2> ], [ <row2_val1>, <row2_val2> ] ] }
 ```
 
+> **Always include `datarows` assertions** — verifying only `total` and `schema` will miss
+> wrong values. Count the expected output groups carefully (e.g., for `chart ... by <col>`,
+> count distinct (row_split, col_split) groups after null filtering, not the number of input rows).
+
 ---
 
 ## Symptom → Fix Path