ISE on coalesce operator (`??`) between named tuples in update

Coalescing a named tuple with the existing property value throws an ISE

```edgeql
with
product := '66d52662-6caa-11f0-8b4e-f71195ea4457',
new_price := >(cents := 100, currency := Currency.ARS),
# Can also be received by param which is how i observed it:
# new_price := >$price,

# select product {
# value := new_price ?? .price, # OK
# }

update product
set {
# price := new_price, # OK
price := new_price ?? .price, # <-- ISE
}
```

This raises
```
InternalServerError: a column definition list is redundant for a function returning a named composite type
```

There are [a few issues](https://github.com/geldata/gel/issues?q=is%3Aissue%20column%20definition%20list) about this error, https://github.com/geldata/gel/issues/5905 specifically mentions named tuples.

- Gel Version: 6.9+77bc1f3
- Gel CLI Version: Gel CLI 7.7.0+3788f53
- OS Version: Fedora release 41 (Forty One)

**Steps to Reproduce:**

1. Use schema
2. Insert an entry
3. Run query to update that entry (adjusting uuid)

**Schema:**

Haven't tested this but here's a reduced version of what i have).

```edgeql
using future simple_scoping;

module default {
scalar type PriceCents extending int64 {
constraint min_value(0);
}
scalar type Currency extending enum;

type Product {
required price: tuple;
}
}
```

Start by reproducing the supplied update query with the reduced schema, checking the named-tuple coalesce behavior against the working select example. Compare the parameter and literal forms described in the issue, then confirm that the update completes without the redundant column definition list error.

Written by the indexing model from the issue text.