feat: make extension type wrappers fallible

[thomaschi78]

Closes #260.

What this changes

GeoArrowExtensionTypeFactory::create_type_instance is infallible by the Polars API contract, so when GeoArrowType::from_extension_field fails (incompatible storage type for the named extension), the factory was panicking via .expect(...).

This PR stores the error inside the wrapper type instead of panicking:

• geoarrow.rs — changes the macro-generated structs from struct PointType(T) to struct PointType(Result<T, String>). Adds an invalid(error) constructor and a private format_inner helper (shared by dyn_display and dyn_debug to avoid duplication). serialize_metadata returns None on error. name() always returns the correct constant regardless of validity. Replaces the infallible From<WrapperType> for geoarrow_schema::T with TryFrom.

• factory.rs — replaces .expect(...) with a match; the error arm dispatches by name to create the right wrapper type in invalid state.

Tests (22 new)

Covers the valid and error paths for:

• TryFrom roundtrip and error message preservation
• From infallible constructor
• name() with valid and invalid inner value
• serialize_metadata returning None in both cases
• dyn_display / dyn_debug output for valid and invalid types
• Clone on invalid types
• PartialEq — same-error equal, different-error unequal, valid ≠ invalid
• Factory happy path (valid storage → valid state)
• Factory error path (incompatible storage → invalid state, no panic, correct name)