fix: checkForUpdate reports restartRequired when current patch is rolled back (#312)

Previously, checkForUpdate returned upToDate after a rollback because
the condition `next != null && current?.number != next.number` treated
a null next patch as "up to date". After a rollback, the Rust updater
correctly uninstalls the patch (next becomes null), but the app is still
running the rolled-back patch (current is non-null). The simplified
condition `current?.number != next?.number` correctly detects this
mismatch and returns restartRequired.

Fixes https://github.com/shorebirdtech/shorebird/issues/3206

shorebird_code_push/lib/src/shorebird_updater_io.dart

@@ -81,7 +81,12 @@ class ShorebirdUpdaterImpl implements ShorebirdUpdater {
     // If no new update is available for download, see if a new patch exists
     // on disk that requires a restart.
     final (current, next) = await (readCurrentPatch(), readNextPatch()).wait;
-    return next != null && current?.number != next.number
+    // A restart is required when the current and next patches differ. This
+    // covers both the "new patch downloaded" case (next differs from current)
+    // and the "current patch was rolled back" case (current is non-null but
+    // next is null, meaning the app needs to restart to revert to the base
+    // release).
+    return current?.number != next?.number
         ? UpdateStatus.restartRequired
         : UpdateStatus.upToDate;
   }

shorebird_code_push/test/src/shorebird_updater_io_test.dart

@@ -270,6 +270,22 @@ void main() {
         });
       });
 
+      group('when no patches are installed and none available', () {
+        setUp(() {
+          when(updater.currentPatchNumber).thenReturn(0);
+          when(updater.nextPatchNumber).thenReturn(0);
+          when(updater.checkForDownloadableUpdate).thenReturn(false);
+          shorebirdUpdater = ShorebirdUpdaterImpl(updater: updater, run: run);
+        });
+
+        test('returns UpdateStatus.upToDate', () async {
+          await expectLater(
+            shorebirdUpdater.checkForUpdate(),
+            completion(equals(UpdateStatus.upToDate)),
+          );
+        });
+      });
+
       group('when updater installed an update and is up to date', () {
         setUp(() {
           when(updater.currentPatchNumber).thenReturn(1);
@@ -286,6 +302,24 @@ void main() {
         });
       });
 
+      group('when current patch has been rolled back', () {
+        setUp(() {
+          // The app is currently running patch 1, but checkForDownloadableUpdate
+          // triggered a rollback which set next_boot_patch to None (0).
+          when(updater.currentPatchNumber).thenReturn(1);
+          when(updater.nextPatchNumber).thenReturn(0);
+          when(updater.checkForDownloadableUpdate).thenReturn(false);
+          shorebirdUpdater = ShorebirdUpdaterImpl(updater: updater, run: run);
+        });
+
+        test('returns UpdateStatus.restartRequired', () async {
+          await expectLater(
+            shorebirdUpdater.checkForUpdate(),
+            completion(equals(UpdateStatus.restartRequired)),
+          );
+        });
+      });
+
       group('when a track is provided', () {
         const track = UpdateTrack.beta;