Fix incorrect JOIN/ON clause reconstruction

[Copilot]

Problem

When parsing SQL statements with multiple JOIN clauses, the reconstructed SQL string (Statement.String()) incorrectly ordered the JOIN and ON clauses. For example:

- FROM worker w
- JOIN event_worker ew ON ew.worker_id = w."id"
- JOIN sys_event se ON se."id" = ew."event"
+ FROM "worker" AS "w"
+ JOIN "event_worker" AS "ew"
+ JOIN "sys_event" AS "se" ON "se"."id" = "ew"."event"
+ ON "ew"."worker_id" = "w"."id"

The issue was in how the JoinClause.String() method handled nested JOIN clauses created by the parser.

Solution

This PR updates the JoinClause.String() method to correctly handle the nested structure of JOIN clauses built by the parser. The solution specifically:

1. Handles simple JOINs with a constraint
2. Handles a single level of nested JOINs
3. Handles double-nested JOINs for more complex queries

The changes are minimal and focused on just the JoinClause.String() method. Comprehensive tests were added to verify correct reconstruction of:

• Basic multiple JOINs with ON constraints
• Complex queries with three or more tables
• Mixed JOIN types (LEFT JOIN, INNER JOIN, CROSS JOIN)

Example

The reconstructed SQL now correctly maintains the proper ordering of JOIN and ON clauses:

SELECT * FROM "worker" AS "w" JOIN "event_worker" AS "ew" ON "ew"."worker_id" = "w"."id" JOIN "sys_event" AS "se" ON "se"."id" = "ew"."event"

Fixes #47.

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[otoolep]

@copilot -- run 'go fmt ./...' to format your source code.

[Copilot]

@copilot -- run 'go fmt ./...' to format your source code.

I've run go fmt ./... and committed the formatting changes in f1f7c63.

[coreybutler]

👍