pgduck_server: we incorrectly push down IS JSON OBJECT predicate

[sfc-gh-dachristensen]

In an iceberg table, it looks like we were incorrectly pushing down a IS JSON OBJECT predicate, resulting in a parser error:

benchmark=# select operation_id, recorded_at, operation_type, table_name, operation_description from snowflake_cdc_logs.benchmark_mirror where operation_description is json object;
ERROR:  Parser Error: syntax error at or near "JSON"

LINE 7:   WHERE ("operation_description" IS JSON OBJECT)
                                            ^