fix(subset): deduplicate CTE names when a table is reachable via multiple FK paths

[joshuabaird]

Problem

When the FK graph contains a diamond pattern — a shared dependency reachable from the subset root via two or more distinct FK paths — cteQuery.addItem was called multiple times with the same CTE name. This produced a WITH clause with duplicate names, which PostgreSQL rejects with error 42712:

WITH query name "public__<schema>__<table>__ids" specified more than once

Reproduction

Any schema where a single table is reachable via multiple FK paths from the subset root triggers this. A concrete real-world example using dj-stripe:

• Subset root: djstripe_customer with subset_conds
• djstripe_paymentmethod.customer_id → djstripe_customer (direct path)
• djstripe_invoice.customer_id → djstripe_customer (direct path)
• djstripe_invoice.default_payment_method_id → djstripe_paymentmethod (indirect path)

When Greenmask walks the graph from djstripe_customer, it encounters djstripe_paymentmethod via two routes and calls addItem("public__djstripe_paymentmethod__ids", ...) twice, producing an invalid WITH clause.

I believe this is related to #265.

Fix

Track seen CTE names in a map[string]struct{} on cteQuery and skip addItem if the name is already registered. The first definition wins — both paths produce equivalent content for the same table.

Test plan

• Run greenmask dump against a schema with diamond-shaped FK dependencies and subset_conds on the shared ancestor — confirm no 42712 error
• Confirm existing subset tests still pass

[wwoytenko]

Hey! Thank you for your MR. Could you provide please database schema with data that can reproduce it?