Add S/R tests for retrying once on HTTP 500

Tests are slightly complicated by the Chorus bug where ErrorEncountered
isn't set to true if the only error was during an hg push operation.
Once that Chorus bug is fixed, we can simplify this test a bit.

Author: rmunn

backend/Testing/FwHeadless/Services/SyncWorkerTests.cs

@@ -109,6 +109,62 @@ public async Task ExecuteSync_PostSendReceiveFailure_ReturnsExpectedStatus(strin
             PostSendReceive);
     }
 
+    [Theory]
+    [InlineData(false, true, SyncJobStatusEnum.SendReceiveFailed)]
+    [InlineData(true, true, SyncJobStatusEnum.Success)]
+    // Currently (2026-03) Chorus doesn't set ErrorEncountered to true if the only error is during a push,
+    // so we also need to test that the second S/R will be triggered on HTTP 500 errors even if IProgress.ErrorEncountered is false.
+    // Once Chorus fixes that bug and errors on push are detected, the second bool param can be removed and this test can be simplified
+    [InlineData(false, false, SyncJobStatusEnum.SendReceiveFailed)]
+    [InlineData(true, false, SyncJobStatusEnum.Success)]
+    public async Task ExecuteSync_PostSendReceiveHttp500_RetriesOneTime(bool retrySucceeds, bool progressReportsErrorOnHttp500, SyncJobStatusEnum expectedStatus)
+    {
+        using var h = new SyncWorkerTestHarness();
+        const string error500 = SendReceiveHelpers.LfMergeBridgeResult.Http500Indicator;
+        var srResults = new List<SendReceiveHelpers.LfMergeBridgeResult>();
+        // First S/R succeeds
+        srResults.Add(new SendReceiveHelpers.LfMergeBridgeResult("success"));
+        // Second S/R gets HTTP 500, but Chorus may or may not record that fact
+        srResults.Add(progressReportsErrorOnHttp500
+            ? new SendReceiveHelpers.LfMergeBridgeResult(error500, ProgressHelper.CreateErrorProgress())
+            : new SendReceiveHelpers.LfMergeBridgeResult(error500));
+        if (retrySucceeds)
+        {
+            // "Third" S/R (first and only retry of second S/R) succeeds
+            srResults.Add(new SendReceiveHelpers.LfMergeBridgeResult("success"));
+        }
+        else
+        {
+            // "Third" S/R (first and only retry of second S/R) also gets HTTP 500, but Chorus may or may not record that fact
+            srResults.Add(progressReportsErrorOnHttp500
+                ? new SendReceiveHelpers.LfMergeBridgeResult(error500, ProgressHelper.CreateErrorProgress())
+                : new SendReceiveHelpers.LfMergeBridgeResult(error500));
+        }
+        h.SetSendReceiveResults(srResults.ToArray());
+
+        var syncResult = new SyncResult(CrdtChanges: 5, FwdataChanges: 3);
+        var result = await h.RunAsync(syncResult);
+
+        result.Status.Should().Be(expectedStatus);
+        var expectedSteps = new List<SyncStep>([
+            TestAuth,
+            CheckBlocked,
+            PreSendReceive,
+            MediaSyncFwData,
+            MediaSyncCrdt,
+            GetSnapshot,
+            Sync,
+            PostSendReceive,
+            PostSendReceive
+        ]);
+        if (retrySucceeds)
+        {
+            expectedSteps.Add(RegenerateSnapshot);
+            expectedSteps.Add(HarmonySync);
+        }
+        h.Steps.Should().Equal(expectedSteps);
+    }
+
     [Theory]
     [InlineData("Rolling back... validation error", true, SyncJobStatusEnum.SyncBlocked)]
     [InlineData("network error", false, SyncJobStatusEnum.SendReceiveFailed)]