write/read rescan is rejected inside one Cypher query

- Docker image `apache/age:release_PG18_1.8.0

**Reproduction**

```sql
LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_issue_case2');

SELECT *
FROM ag_catalog.cypher('age_issue_case2', $$
CREATE (a {id: 1})-[:e]->(b {id: 2})
$$) AS (v agtype);

SELECT *
FROM ag_catalog.cypher('age_issue_case2', $$
MERGE ({id: 28})
MERGE p0 = (:l0 {k0: false, id: 29})
WITH tan(toFloat(length(p0))) AS alias0,
range(6, 14) + [0] AS alias1,
p0 AS alias2
WHERE true
MATCH p1 = (n0) -[*1..1]-()
RETURN collect(toString(n0.id)) AS alias3
SKIP 0 LIMIT 9
$$) AS (alias3 agtype);
```

**Expected**

The valid Cypher query executes and returns the matching path result.

**Actual**

```text
cypher merge clause cannot be rescanned
HINT: its unsafe to use joins in a query with a Cypher MERGE clause
```

The corresponding `CREATE` variant reports `cypher create clause cannot be
rescanned` when the later graph read finds a matching path.

Start by running the supplied SQL reproduction against Docker image apache/age:release_PG18_1.8.0, then compare the MERGE and CREATE cases with their expected graph-read results. Done means the valid write/read query executes and returns the matching path instead of reporting that the clause cannot be rescanned.

Written by the indexing model from the issue text.