[PATCH] Fix segmentation fault and infinite loop in jsonb_{plperl,plpython}

Hi,

I discovered several bugs in jsonb_plperl and jsonb_plpython.

The first bug causes a segfault when dealing with deeply nested JSONB
values. As an example:

```
$ ./reproduce_stack_overflow.py
plpython3u (depth=100000): SIGSEGV
2026-06-16 16:42:56.989 MSK [3209763] LOG: client backend (PID
3209810) was terminated by signal 11: Segmentation fault
2026-06-16 16:42:56.989 MSK [3209763] DETAIL: Failed process was
running: SELECT py_deep(100000);
plperl (depth=100000): SIGSEGV
2026-06-16 16:42:59.101 MSK [3209763] LOG: client backend (PID
3209827) was terminated by signal 11: Segmentation fault
2026-06-16 16:42:59.101 MSK [3209763] DETAIL: Failed process was
running: SELECT perl_deep(100000);
```

The second bug affects only jsonb_plperl. It's possible to construct a
Perl object with circular references which will cause
SV_to_JsonbValue() to go into an infinite loop here:

```
while (SvROK(in))
in = SvRV(in);
```

The attached script reproduce_circular_ref.py reproduces the issue. Be
careful if you decide to run it because the backend will become
unresponsive to pg_cancel_backend() and you will be unable to stop the
cluster in a standard way.

I suggest fixing it by rewriting the while loop into a recursion with
check_stack_depth() call. This will make the behavior consistent with
jsonb_plpython.

Patches are attached. Thoughts?

--
Best regards,
Aleksander Alekseev