fix(splitter): support CTE column lists in CREATE ... AS

[edjubert]

What

The splitter's cte rule expected IDENT AS (. A CTE may also declare its output columns before AS, and that form made the rule bail out, so the statement was cut in two before the final SELECT:

CREATE TABLE ranked_items AS
WITH
    priorities (label, rank) AS (
        VALUES
            ('high', 1),
            ('medium', 2),
            ('low', 3)
    )
SELECT
    items.id,
    priorities.rank
FROM items
JOIN priorities ON priorities.label = items.priority;

Before this fix, everything up to the closing parenthesis of the CTE was submitted to the parser on its own, which reported syntax error at end of input on a perfectly valid file.

Implementation

Three lines in splitter/dml.rs: consume an optional parenthesised column list between the CTE name and AS.

Why here

This extends fix(splitter): keep create-as CTEs attached, whose test only covers a CTE whose body is a SELECT.
The column-list form - most common with VALUES lookup tables — was not covered.

Tests

create_as_with_values_cte in crates/pgls_statement_splitter/src/lib.rs, using the exact statement above, asserting two statements and no errors.