typing: fix `1.4.1` stubs

[dangotbanned]

Originally posted by @dangotbanned in #124

Downstream in narwhals, I've got a PR open which is trying to adjust to the new stubs:

• fix(DONT MERGE): duckdb>=1.4.1 typing & warnings narwhals-dev/narwhals#3189

From #96 the issues we have seem to stem from (236713a) and (f5d4e42) after which we get a lot of false negatives on:

• ConstantExpression
• LambdaExpression
• DuckDBPyType

I think the fixes I have for ConstantExpression + LambdaExpression would be pretty minor.

There's 1 or 2 examples for ConstantExpression in the test suite already 😉

Show ConstantExpression

DuckDBPyType might be a little more involved, since it's an issue with children.
Also the way I've worked around it in narwhals probably wouldn't be the same thing I'd write in the stubs

[evertlammerts]

Fixed in #323. The children property was typed as list[tuple[str, object]] which gave callers no useful type information. Changed it to list[tuple[str, DuckDBPyType | int | list[str]]] to reflect what the C++ implementation actually returns (DuckDBPyType for nested types, int for DECIMAL precision/scale and ARRAY size, and list[str] for ENUM values).

Note: the ConstantExpression and LambdaExpression issues from the original report were already resolved in a previous release when their parameter types were changed to Any.