Fix select_dtypes for quoted column identifiers

[Schwarf]

Fixesduckdb/duckdb#20329

DuckDBPyRelation.select_dtypes failed for relations with column names that
require quoting (e.g., names containing spaces). The projection builder did
not correctly quote identifiers, leading to binder errors.

This change ensures identifiers are quoted consistently and adds a regression
test to cover the reported case.

[evertlammerts]

Thanks! DuckDB has a helper you can use for this: duckdb::KeywordHelper::WriteOptionallyQuoted. Will that suffice?

[Schwarf]

Thanks! DuckDB has a helper you can use for this: duckdb::KeywordHelper::WriteOptionallyQuoted. Will that suffice?

Thanks! I’ve switched to duckdb::KeywordHelper::WriteOptionallyQuoted as suggested and removed the custom identifier-quoting helper. The regression test continues to pass.

[evertlammerts]

Perfect, thanks! I'll merge without running the tests - main is currently broken.