test(stack): re-port searchable-json-pg tests from protect after 0.23.0

The original port (commit 643785c) was based on the pre-0.23.0 protect
test file. #473 substantially rewrote protect's searchable-json-pg
tests to match protect-ffi 0.23.0's stricter HMAC/shape validation
and changed selector/queryType semantics; 37 of the ported tests
were left running on the old EQL contract and failed.

Re-port from the current protect file and re-apply the stack-specific
adjustments: imports from @/identity, @/index, @/schema; encryptedTable
/ encryptedColumn instead of csTable / csColumn; protect-ci-jsonb-stack
table name to avoid collision with protect's suite; cast null in the
null round-trip test since stack's encrypt() drops null from its
JsPlaintext input by design.

Author: coderdan