test: cover pandas 2 ingestion paths

[j6k4m8]

Summary

• cover populated DynamoDB dataframe ingestion with isolated batch-writer mocks
• verify DynamoDB emits complete edge and node rows without initializing cloud clients
• cover empty dataframe ingestion for DynamoDB, NetworkX, and SQL
• prevent regressions to removed pandas Series append APIs

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

---

_{Comparing test/pandas2-ingestion-72 (63daf14) with master (7c0d2e8)}