mysql_cdc: ensure snapshot primary key order is consistent for composite primary keys

[josephwoodward]

Go's map do not guarantee ordering (the Go team even intentionally build randomisation into it to dissuade people from depending on its order), so switch to iterating ordered pk slice instead.

Closes: #4256

[claude]

Commits
LGTM

Review
Clean bug fix: querySnapshotTable was iterating over a map[string]any (*lastSeenPkVal) directly, which yields non-deterministic key order in Go. For composite primary keys, this caused the values passed to the WHERE (col1, col2) > (?, ?) clause to potentially be in the wrong order relative to the column names from pk, resulting in incorrect snapshot pagination. The fix iterates over the ordered pk slice and looks up each value by name, ensuring consistent column-value alignment. Covered by existing integration test TestIntegrationMySQLCDCWithCompositePrimaryKeys.

LGTM

[claude]

Commits
LGTM

Review
Single-file bug fix in internal/impl/mysql/snapshot.go that switches snapshot pagination from iterating over a map (non-deterministic order in Go) to iterating over the pk slice, ensuring primary key column values are always in the correct order when constructing the paginated query. The missing-key error path is handled properly.

LGTM

[mmatczuk]

LGTM