Potential Memory Leak with `jsonb_build_object()` on Columnar Tables

PosgreSQL's `jsonb_build_object()` over columnar tables consumes excessive memory and leads to OOM crash.
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Hi team,
I'm running into what appears to be a memory leak when executing a GROUP BY query using jsonb_build_object() over a few columnar tables on:
- PostgreSQL: 16
- Citus: 13
- Hosted on a single EC2 instance with ~250 GB RAM.

### Symptoms:
The query runs for ~3 minutes, consuming:
- **155GB RES**
- **290GB VIRT**
- **63%+ of total memory**

Once memory usage crosses 206GB+, the server eventually crashes with an OOM error if not manually killed.
The query itself is a fairly typical OLAP-style aggregation.


### Query Summary:
The pattern (simplified) looks like:
```
WITH filtered_data AS (
SELECT ...
FROM columnar_table_1 AS a
JOIN columnar_table_2 AS b ON ...
LEFT JOIN columnar_table_3 AS c ON ...
WHERE c.some_json_field ->> 'some_key' = 'some_value'
)
SELECT
jsonb_build_object(...),
some_keys,
SUM(...),
VARIANCE(...)
FROM filtered_data
GROUP BY ...;
```

- All input tables are columnar.
- Two of the inputs are joined on multiple fields, and the main aggregation is done after a `jsonb_build_object()` call in the `SELECT`.

### Observations:
- The problem only occurs when `jsonb_build_object()` is used.
- No UDFs or temporary tables are involved.
- Setting `max_parallel_workers_per_gather = 0` and disabling `columnar.enable_custom_scan` doesn't resolve the issue.

### Question:
1. Is this a known limitation or a bug in how Citus handles `jsonb_build_object()` over columnar data with parallel execution or row-materialization?
2. Any recommendations on safely constructing JSON from large columnar datasets?