fix: SQL quality rules on mysql/iceberg read as DuckDB SQL

[jschoedl]

SQL quality rules on mysql and iceberg servers are now parsed by the read-only SQL guard as DuckDB SQL, which is what actually runs them. That closes a parser-differential smuggling hole. On the mysql path, the attached MySQL catalog is DETACHed once tables are copied, and untrusted contracts get restrict_to_paths(con, []). Double-quotes in table names are escaped to prevent SQL injection through physicalName.

Fixes:

• Parser differential where a MySQL backslash escape hides a second statement from the guard
• A posted contract reaching MySQL with environment credentials via malicious physicalName
• A posted contract reading the MySQL connection string (password included) via duckdb_databases()