Fix segmentation fault with MATCH inside CTEs and UNION (issue #276)

[nicolas-geysse]

Summary

• Fixes crash when using GRAPH_TABLE inside CTEs or with UNION operations
• Adds recursive query node traversal for SetOperationNode
• Adds regression tests for CTE and UNION scenarios

Problem

The parser extension's duckpgq_find_select_statement() was not handling SetOperationNode (UNION/UNION ALL/EXCEPT/INTERSECT), causing segfaults when GRAPH_TABLE was used in these contexts:

-- Crash with CTE containing GRAPH_TABLE
WITH mfp AS (
  FROM GRAPH_TABLE (snb MATCH (a:Person)-[k:knows]->(b:Person) COLUMNS (...))
  SELECT ...
)
FROM mfp, GRAPH_TABLE (snb MATCH ...) mem
SELECT ...

-- Crash with UNION
SELECT ... FROM GRAPH_TABLE(...)
UNION ALL
SELECT ... FROM GRAPH_TABLE(...)

Error: INTERNAL Error: Attempted to dereference unique_ptr that is NULL!

Solution

Added a recursive duckpgq_traverse_query_node() function that properly handles all QueryNode types:

• SelectNode: traverse from_table and CTEs
• CTENode: recurse into child node
• SetOperationNode: recurse into all children (for UNION/EXCEPT/INTERSECT)

Test plan

• New test file test/sql/cte_union_match.test with 5 test cases
• All 126 existing tests pass
• Tests cover: single CTE, multiple CTEs, UNION ALL, CTE + main GRAPH_TABLE, UNION with different patterns

Fixes #276

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