`abdc_ffi`: Cancelled in-flight queries cannot report `ECANCELED`

The rust `abdc_ffi` driver exporter exports result readers via arrow-rs's `FFI_ArrowArrayStream::new`, whose error→errno mapping is hardcoded to the `ArrowError` **variant**: `NotYetImplemented`→`ENOSYS`, `MemoryError`→`ENOMEM`, `IoError`→`EIO`, everything else→`EINVAL`.

Rust ADBC drivers surface their `adbc_core::error::Error` through `ArrowError::ExternalError`, which lands in the `EINVAL` catch-all — so a driver whose in-flight query was cancelled has **no way** to report `ECANCELED` through the C stream, even though the ADBC spec (and the C++ validation suite's `SqlQueryCancel`) expect exactly that errno after `AdbcStatementCancel`.

### Environment/Setup
`adbc_ffi` 0.23.0 (also present in current `main`)