Server crashes (segfault) on `cypher(CREATE ...)` inside SQL `WHERE NOT EXISTS` with a nested `cypher(MATCH ...)` subquery

**Bug description**

Writing a Cypher `CREATE` in the outer query of a `SELECT ... WHERE NOT EXISTS (...)` statement, where the `NOT EXISTS` subquery contains another `cypher(MATCH ...)` call, crashes the PostgreSQL backend with a segmentation fault. The backend process is terminated (`signal 11: Segmentation fault`) and the connection is dropped.

**Access method**

- Command line via `psql`, inside the official Docker container `apache/age:1.8.0`

**Data setup**

No data is required — the crash reproduces on an empty graph. Only the graph itself must exist:

```pgsql
CREATE EXTENSION IF NOT EXISTS age;
LOAD 'age';
SET search_path = ag_catalog, "$user", public;
SELECT create_graph('graph_test');
```

**Configuration**

- None beyond the stock AGE extension. No additional modules (no PostGIS, etc.), default `search_path` handling as shown above.

**Command that triggers the crash**

```pgsql
SELECT * FROM cypher('graph_test', $$ CREATE () $$) AS (a agtype)
WHERE NOT EXISTS (SELECT * FROM cypher('graph_test', $$ MATCH () RETURN 1 $$) AS (b agtype));
```

```
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.
Time: 2171.760 ms (00:02.172)
```

The server log shows the backend being killed:

```
LOG: client backend (PID NNN) was terminated by signal 11: Segmentation fault
LOG: terminating any other active server processes
LOG: all server processes terminated; reinitializing
```

**Expected behavior**

The statement is a legal combination of two individually-valid Cypher queries glued together with SQL's `WHERE NOT EXISTS`. It should either execute normally (creating the node and returning a row) or raise a normal SQL error — it must not crash the backend process.

**Environment**

- Version: 1.8.0 (official `apache/age:1.8.0` Docker image)
- PostgreSQL: 18.1 (Debian 18.1-1.pgdg13+2), x86_64

Reproduce the crash with psql in the official apache/age:1.8.0 Docker container using the provided graph setup and nested cypher query. Compare the observed backend termination with the expected normal execution or SQL error; the issue names no source files or tests, so locating the failing PostgreSQL extension path will require repository research.

Written by the indexing model from the issue text.