Summary

Changes the return type of OutputStream::write_all_async/write_all_future and the symmetric InputStream::read_all_async/read_all_future/read_all from Result<(B, usize, Option<glib::Error>), (B, glib::Error)> to Result<(B, usize), (B, usize, glib::Error)>, so callers no longer inspect an inner Option on the Ok path and the error path carries the partial byte count.

Why this matters

The old Ok-with-Some(error) shape forced every caller to check the inner Option even on success, which is confusing and easy to get wrong (#1942). @sdroege agreed the design is wrong and stated the preferred variant Result<(B, usize), (B, usize, glib::Error)>, which also surfaces the partial count on failure. Each async trampoline now builds Ok((buffer, bytes_written)) when no error occurred and Err((buffer, bytes_written, ...)) otherwise, dropping the prior three-state branch. The *_future wrappers mirror the new callback type and the sync read_all surfaces the error directly.

This is a breaking API change, appropriate for the in-progress 0.23.0-alpha cycle.

Testing

• Added cancellation tests asserting the error path carries the partial byte count for both read_all and write_all.

Closes #1942