Full-path `MERGE` followed by a `WITH` constant `STARTS WITH` predicate causes a PostgreSQL backend segmentation fault

Docker image `apache/age:release_PG18_1.7.0

**Reproduction**

```sql
LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_issue_case10');

SELECT *
FROM ag_catalog.cypher('age_issue_case10', $$
MERGE p = (:a)-[:r]->(n)
WITH 1 AS y
WHERE ('a' STARTS WITH 'a')
RETURN y
$$) AS (y agtype);
```

**Expected**

The query returns one row containing `1` and PostgreSQL remains available.

**Actual**

```text
server closed the connection unexpectedly
client backend ... was terminated by signal 11: Segmentation fault

Reproduce the SQL through ag_catalog.cypher using the apache/age:release_PG18_1.7.0 Docker image, then trace handling of the MERGE/WITH/STARTS WITH combination in the C/PostgreSQL extension. Done means the query returns one row containing 1, PostgreSQL remains available, and regression coverage verifies that behavior.

Written by the indexing model from the issue text.