Implement `adbc_get_table_schema` for Polars

### What feature or improvement would you like to see?

As of [Polars 1.33.1](https://github.com/pola-rs/polars/releases/tag/py-1.33.1) `__arrow_c_schema__` was introduced on Polars.Schema (https://github.com/pola-rs/polars/pull/24365)

As of [Polars 1.32.1](https://github.com/pola-rs/polars/releases/tag/py-1.32.1) (which was yanked, so really 1.32.2), `Polars.Schema` can be initialised from an object with `__arrow_c_schema__` (https://github.com/pola-rs/polars/pull/23879)

I believe this means `adbc_get_table_schema` can be implemented for the Polars backend (correct me if mistaken).

Ref https://github.com/apache/arrow-adbc/pull/2839#discussion_r2100047698

Start at the Polars backend implementation of `adbc_get_table_schema` and review the linked Arrow ADBC PR discussion, then check how Polars 1.32.2 exposes `__arrow_c_schema__`. Done means the method returns the table schema correctly for supported Polars versions, with the backend's existing tests covering the behavior.

Written by the indexing model from the issue text.