fix(ai): treat rewind at end of history as no-op success

The mobile client's regenerate-after-failure flow sends a discard index
equal to the server's rendered count (its optimistic user bubble for the
failed turn was never persisted). find_raw_cut treated this as out of
range, surfacing as "Chat rewind failed: discard_from_rendered_index out
of range" and blocking the retry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ai/insight_chat.rs

@@ -1071,8 +1071,13 @@ fn is_rendered(m: &ChatMessage) -> bool {
 /// Given a rendered index to start discarding from, find the raw index at
 /// which to truncate. The cut position is the raw length after all prior
 /// rendered messages — which also strips any tool-call scaffolding that
-/// immediately precedes the discarded rendered message. Returns `None` if
-/// `discard_from_rendered_index` is past the end of the rendered view.
+/// immediately precedes the discarded rendered message.
+///
+/// Discarding *at* the end (`discard == rendered_count`) is a no-op success:
+/// returns `Some(messages.len())`. The mobile client hits this when
+/// regenerating after a failed turn — its optimistic user bubble lives at
+/// the index just past the server's persisted history. Strictly past the end
+/// (`discard > rendered_count`) returns `None`.
 pub(crate) fn find_raw_cut(
     messages: &[ChatMessage],
     discard_from_rendered_index: usize,
@@ -1089,10 +1094,8 @@ pub(crate) fn find_raw_cut(
         rendered_count += 1;
         last_kept_raw_end = i + 1;
     }
-    if rendered_count == discard_from_rendered_index {
-        // Discarding past the last rendered message is a no-op, but we
-        // surface it as "nothing to cut" rather than silent success.
-        return None;
+    if discard_from_rendered_index == rendered_count {
+        return Some(messages.len());
     }
     None
 }
@@ -1361,4 +1364,18 @@ mod tests {
         let msgs = vec![ChatMessage::user("q1"), assistant_text("a1")];
         assert!(find_raw_cut(&msgs, 5).is_none());
     }
+
+    #[test]
+    fn rewind_at_end_is_noop_success() {
+        // Mobile client retries after a failed turn that never persisted —
+        // its optimistic user bubble's index equals the server's rendered
+        // count. Should resolve to "no cut" rather than an out-of-range error.
+        let msgs = vec![
+            ChatMessage::system("s"),
+            ChatMessage::user("q1"),
+            assistant_text("a1"),
+        ];
+        let cut = find_raw_cut(&msgs, 2).expect("boundary cut should succeed");
+        assert_eq!(cut, msgs.len());
+    }
 }