edit prediction: Report early-rejected predictions and fix cancel bug (#43585)

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

---------

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

crates/zeta_cli/src/predict.rs

@@ -235,7 +235,10 @@ pub async fn perform_predict(
     let mut result = Arc::into_inner(result).unwrap().into_inner().unwrap();
 
     result.diff = prediction
-        .and_then(|prediction| prediction.edit_preview.as_unified_diff(&prediction.edits))
+        .and_then(|prediction| {
+            let prediction = prediction.prediction.ok()?;
+            prediction.edit_preview.as_unified_diff(&prediction.edits)
+        })
         .unwrap_or_default();
 
     anyhow::Ok(result)