Many prediction requests end up being rejected early without ever being
set as the current prediction. Before this change, those cases weren’t
reported as rejections because the `request_prediction_with_*` functions
simply returned `Ok(None)`.

With this update, whenever we get a successful response from the
provider, we will return at least the `id`, allowing it to be properly
reported. The request now also includes a “reject reason,” since the
different variants carry distinct implications for prediction quality.

All of these scenarios are now covered by tests. While adding them, I
also found and fixed a bug where some cancelled predictions were
incorrectly being set as the current one.

Release Notes:

- N/A

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Co-authored-by: MrSubidubi <dev@bahn.sh>