PostgreSQL: Support `PRIMARY KEY USING INDEX` syntax in `ALTER TABLE`

The parser fails to parse the PostgreSQL-specific `PRIMARY KEY USING INDEX index_name` syntax in `ALTER TABLE` statements:

```sql
ALTER TABLE version_downloads
ADD CONSTRAINT version_downloads_pkey PRIMARY KEY USING INDEX version_downloads_unique;
```

```
Expected: a list of columns in parentheses, found: INDEX
```

## Expected behavior

This is valid PostgreSQL syntax per the [ALTER TABLE documentation](https://www.postgresql.org/docs/current/sql-altertable.html). The grammar allows:

```
ADD table_constraint_using_index
```

Where `table_constraint_using_index` can be:

```
[ CONSTRAINT constraint_name ]
{ UNIQUE | PRIMARY KEY } USING INDEX index_name
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
```

Note that `PRIMARY KEY USING INDEX index_name` is a distinct form that does **not** require a column list - it promotes an existing unique index to be the primary key constraint.

先定位 Rust SQL 解析器中的 ALTER TABLE 文法以及資料表約束的處理方式。使用此 issue 中的 PostgreSQL 範例作為解析輸入，接著為 PRIMARY KEY USING INDEX 新增涵蓋範圍，並確認該陳述式不需要欄位清單即可解析。

由索引模型根據 Issue 內容生成。