[core] Reject dot-paths in fields.* options on row columns

[ArnavBalyan]

Purpose

• fields.<row>.<inner>.aggregate-function is silently ignored at write time, the parent row overwrites instead of aggregating. (during write time)
• Reject the misconfig at ddl when the parent field is a RowType. Ensure we can strictly reject this case instead of allowing silent overwrites.
• Fixes: [Bug] Paimon ROW Type Internal Field Sum Aggregation Not Working #6471

Tests

• UT

[ArnavBalyan]

cc @JingsongLi thanks!