Only apply non-left-associative nesting to deferred join constraints

[revitalkr]

Summary

For dialects with supports_left_associative_joins_without_parens = false, nested joins are currently created even when no deferred join constraint exists.

Problem

The non-left-associative join handling was introduced to support deferred join constraints such as:
A JOIN B JOIN C ON X ON Y
However, the same logic is also applied to queries such as:
A JOIN B LEFT JOIN C ON X
even though there is no deferred ON or USING constraint to attach.
This produces a nested join structure where a flat join chain is expected.

Solution

Only apply the nesting logic when a deferred join constraint (ON or USING) remains to be attached.
Queries without deferred constraints continue to use the normal flat join structure.

Test

Adds coverage for:
SELECT 'ORIGINAL' AS src,
o.order_id,
c.customer_id,
p.product_id
FROM orders AS o
JOIN customers AS c
LEFT JOIN products AS p
ON p.order_id = o.order_id
and verifies that both Generic and Snowflake dialects produce a flat join chain (joins.len() == 2).

Validation - Snowflake script

The following query executes successfully in Snowflake:

SELECT 'ORIGINAL' AS src,
o.order_id,
c.customer_id,
p.product_id
FROM orders AS o
JOIN customers AS c
LEFT JOIN products AS p
ON p.order_id = o.order_id;
and matches the flat interpretation:
(orders AS o JOIN customers AS c)
LEFT JOIN products AS p
ON p.order_id = o.order_id

while the nested interpretation generated by the current non-left-associative handling:
orders AS o
JOIN (
customers AS c
LEFT JOIN products AS p
ON p.order_id = o.order_id
)
is rejected by Snowflake.