Build a ValueDecodeError, logging the schema, table, column, source
gtid_set (if any), and a shape description of row at the same time.
The shape string is only built here — pack_mysql_row’s happy path does no
per-row allocation beyond what decoding requires.
Describes the structural shape of a row without revealing any data values.
Iterates every wire column. For each, emits the wire name, the binlog
wire type, the character-set id (or binary), a classification relative
to table_desc (expected=<scalar> for active columns, ignored for
columns excluded from the source, extra for upstream columns with no
descriptor entry), and a value disposition (null or bytes(len=N) /
primitive kind). Intended for diagnostic logging on decode errors: MySQL
serializes CHAR, VARCHAR, TEXT, JSON, BLOB, etc. all as Value::Bytes,
so the wire type tag and the expected scalar type are what distinguish
them.