Function validate_read_only_with_dialect 

pub fn validate_read_only_with_dialect(
    sql: &str,
    dialect: &impl Dialect,
) -> Result<(), AppError>

Validates that a SQL query is read-only.

Parses the query using the given dialect and checks:

1. Exactly one statement (multi-statement injection blocked)
2. Statement type is read-only (SELECT, SHOW, DESCRIBE, USE, EXPLAIN)
3. No dangerous functions (LOAD_FILE)
4. No INTO OUTFILE/DUMPFILE clauses

Errors

Returns AppError if the query is not allowed in read-only mode.