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/cloud_llm_client/src/cloud_llm_client.rs

@@ -200,12 +200,31 @@ pub struct RejectEditPredictionsBody {
     pub rejections: Vec<EditPredictionRejection>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct EditPredictionRejection {
     pub request_id: String,
+    #[serde(default)]
+    pub reason: EditPredictionRejectReason,
     pub was_shown: bool,
 }
 
+#[derive(Default, Debug, Clone, Copy, Serialize, Deserialize, PartialEq)]
+pub enum EditPredictionRejectReason {
+    /// New requests were triggered before this one completed
+    Canceled,
+    /// No edits returned
+    Empty,
+    /// Edits returned, but none remained after interpolation
+    InterpolatedEmpty,
+    /// The new prediction was preferred over the current one
+    Replaced,
+    /// The current prediction was preferred over the new one
+    CurrentPreferred,
+    /// The current prediction was discarded
+    #[default]
+    Discarded,
+}
+
 #[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum CompletionMode {