DMLEvent serialization drops row checksum metadata

[wk989898]

What did you do?

1. Enabled tidb_enable_row_level_checksum=true on upstream TiDB before writing data.
2. Created a changefeed with:

[filter]
rules = ["test.*"]

[integrity]
integrity-check-level = "correctness"
corruption-handle-level = "error"

What did you expect to see?

The decoded event preserves the checksum entries and all their fields: Current, Previous, Version and Corrupted.

What did you see instead?

output.Checksum is nil.

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

master

[wk989898]

Cause and impact

• DMLEvent.encodeV1()/decodeV1() encode/decode event metadata, row types and row keys, but omit Checksum.

• BatchDMLEvent.encodeV1() calls each DML event’s Marshal() and separately encodes the row chunk. It does not preserve checksum metadata
  elsewhere.

• Cross-node messaging uses this serialization. Same-process channel delivery does not trigger this loss.

This affects checksum metadata for INSERT, UPDATE and DELETE on the serialized DML path. Even when a checksum is present before transmission, the receiving encoder cannot use it for Avro/Simple checksum output. This is independent of whether integrity configuration is propagated correctly.