Refactor upsert/connectOrCreate helpers and add integration tests

[prageeth]

Summary

• Extracted shared processUpsertItems and processConnectOrCreateItems helpers to deduplicate upsert and connectOrCreate processing logic across CASE 1 and CASE 2 in processNestedData
• Fixed connectOrCreate in CASE 1 to handle both single-object and array forms (previously only CASE 2 handled arrays)
• Added integration tests for array upserts, nested creates inside upsert update branches, and single-object connectOrCreate across SQLite, MySQL, and PostgreSQL