MERGE INTO PostgreSQL table does not apply target column defaults for omitted columns

### What happens?

When using the PostgreSQL extension, `MERGE INTO` fails to apply PostgreSQL column defaults for omitted columns during the `INSERT` branch. This does not happen with a plain `INSERT` statement.

### To Reproduce

The target table has an autoincrement `id` column with an underlying sequence (in postgres).

SQL steps are executed using the Python client like so:
```python
import duckdb

mem = duckdb.connect()
mem.sql("ATTACH '/path/to/my/duckdb.duckdb' AS duck;")
mem.sql("ATTACH 'host=127.0.0.1 port=5432 user=fbftooling dbname=my_db password=...' AS pg (TYPE postgres);")

mem.sql("INSERT INTO ...")
```

An `INSERT` omitting the autoincrement `id` column from DuckDB works correctly:

```sql
INSERT INTO pg.my_table(name, ...)
SELECT name, ...
FROM duck.my_table;
```

PostgreSQL generates the `id` values as expected.

However, the equivalent `MERGE` fails:

```sql
MERGE INTO pg.my_table AS t
USING duck.my_table AS s
ON lower(t.name) = lower(s.name)
WHEN NOT MATCHED THEN
INSERT (name, ...)
VALUES (s.name, ...);
```

This results in:

```text
ERROR: null value in column "id" of relation "my_table" violates not-null constraint
CONTEXT: COPY my_table, line 1
```

It appears that `MERGE` sends an explicit `NULL` for omitted columns instead of allowing PostgreSQL to apply the column default.

Attempting to explicitly use the PostgreSQL sequence also fails:

```sql
INSERT (id, name)
VALUES (nextval('my_table_id_seq'), s.name)
```

DuckDB reports:

```text
Catalog Error: Sequence with name my_table_sequence_id does not exist!
```

This indicates that `nextval()` is evaluated by DuckDB rather than PostgreSQL, so it is not a viable workaround.

**Expected behavior**

`MERGE` should behave consistently with `INSERT`:

* omitted target columns should use PostgreSQL defaults,
* or otherwise preserve PostgreSQL's normal `INSERT` semantics for default values.

### OS:

Windows 11 (WSL 2)

### PostgreSQL Version:

17.10

### DuckDB Version:

1.5.2

### DuckDB Client:

Python

### Full Name:

Leonardo Pedri

### Affiliation:

Innerspace

### Have you tried this on the latest `main` branch?

- [x] I agree

### Have you tried the steps to reproduce? Do they include all relevant data and configuration? Does the issue you report still appear there?

- [x] I agree

Reproduce the MERGE and plain INSERT comparison using the Python client with attached DuckDB and PostgreSQL databases. Trace the PostgreSQL extension's MERGE INSERT branch and add a regression test showing that omitted target columns receive PostgreSQL defaults; done when MERGE matches INSERT without evaluating PostgreSQL sequences in DuckDB.

Written by the indexing model from the issue text.