Function merge_changelog 

pub async fn merge_changelog(
    table: DeltaTable,
    source_batch: RecordBatch,
    key_columns: &[String],
    writer_id: &str,
    epoch: u64,
    schema_evolution: bool,
) -> Result<(DeltaTable, MergeResult), ConnectorError>

Atomic changelog MERGE: inserts, updates, and deletes in one Delta commit.

The source batch must contain an _op column (Utf8) with values:

• "I", "U", "r" → upsert (update if matched, insert if not)
• "D" → delete matched rows

Columns prefixed with _ are excluded from SET clauses but remain in the source DataFrame for predicate filtering.

Errors

Returns ConnectorError::WriteError if the merge fails.