`DIV` right operand is parsed at precedence 0 in the MySQL and Spark dialects

[zvonimir-dd]

DIV is announced at Precedence::MulDivModOp by get_next_precedence, but the MySQL and Spark
dialects parse its right operand with parse_expr() (= parse_subexpr(0)), so the operand absorbs
every following operator instead of stopping at DIV's own precedence.

SQL	Parsed as	MySQL / Spark
7 DIV 2 + 1	7 DIV (2 + 1) = 2	(7 DIV 2) + 1 = 4
9 DIV 3 * 3	9 DIV (3 * 3) = 1	(9 DIV 3) * 3 = 9
a DIV 2 = 1	a DIV (2 = 1)	(a DIV 2) = 1

Both systems place DIV with * and /: MySQL's
operator precedence table lists
*, /, DIV, %, MOD on one row, and Spark's SqlBaseParser.g4 has
operator=(ASTERISK | SLASH | PERCENT | DIV) in a single left-recursive rule.

Display for Expr::BinaryOp emits no parentheses, so a mis-grouped tree round-trips back to the
original SQL — verified_expr / verified_stmt cannot catch this, which is why it has gone
unnoticed. Only a test that asserts on the tree will.

Fix

Thread the precedence argument through, exactly as SqliteDialect::parse_infix already does for
REGEXP / MATCH / GLOB (#2419):

--- a/src/dialect/mysql.rs
+++ b/src/dialect/mysql.rs
-        _precedence: u8,
+        precedence: u8,
-            let right = Box::new(match parser.parse_expr() {
+            let right = Box::new(match parser.parse_subexpr(precedence) {

--- a/src/dialect/spark.rs
+++ b/src/dialect/spark.rs
-        _precedence: u8,
+        precedence: u8,
-            let right = Box::new(match parser.parse_expr() {
+            let right = Box::new(match parser.parse_subexpr(precedence) {

Red test

#[test]
fn parse_div_precedence() {
    // `DIV` has the same precedence as `*` and `/`, so `+` must end up at the root.
    assert_eq!(
        Expr::BinaryOp {
            left: Box::new(Expr::BinaryOp {
                left: Box::new(Expr::value(number("7"))),
                op: BinaryOperator::MyIntegerDivide,
                right: Box::new(Expr::value(number("2"))),
            }),
            op: BinaryOperator::Plus,
            right: Box::new(Expr::value(number("1"))),
        },
        mysql().verified_expr("7 DIV 2 + 1")
    );
}

Credit to @LucaCappelletti94, who spotted this and wrote both the patch and the test while reviewing
#2436. I'll open a PR once #2436 lands, to keep the two precedence changes reviewable separately —
happy for someone else to pick it up sooner.