fix: scope chat access policies to schema-qualified tables

Schema-scoped policies (e.g. marts-only amount redaction) previously matched
bare column names across all schemas and blocked SELECT * globally. Protection
descriptors are now keyed by schema.table, allowed-schema lists are parsed from
policy text and enforced at SQL pre-check, and numeric amount denials respect
the allowed schema scope.

Co-authored-by: Venkat SF <venkatesh.sakamuri@stayflexi.com>

Author: cursoragent