Always emit model usage array

Author: flin-8

source/Calamari.Tests/AiAgent/ClaudeCodeBehaviour/ClaudeCodeUsageReporterFixture.cs

@@ -107,6 +107,22 @@ public void RunSummaryAttributes_ArePreservedOnTheMessage()
         msg.GetValue(ClaudeCodeServiceMessages.Usage.ModelUsageAttribute).Should().NotBeNull();
     }
 
+    [Test]
+    public void RunSummaryWithoutModelUsage_StillCarriesAnEmptyModelUsageAttribute()
+    {
+        // The server requires the attribute; a run that fails before any model round-trip has none
+        reporter.SetRunSummary(new ClaudeCodeRunSummary
+        {
+            TotalCostUsd = 0,
+            DurationMs = 1234,
+            NumTurns = 0,
+        });
+
+        reporter.WriteServiceMessage(log);
+
+        DeserializeModelUsage().Should().BeEmpty();
+    }
+
     ClaudeCodeModelUsage[] DeserializeModelUsage()
     {
         var msg = log.ServiceMessages.Single(m => m.Name == ClaudeCodeServiceMessages.Usage.Name);

source/Calamari/AiAgent/ClaudeCodeBehaviour/ClaudeCodeUsageReporter.cs

@@ -44,14 +44,13 @@ public void WriteServiceMessage(ILog log)
     {
         if (modelUsage.Count == 0 && summaryProperties.Count == 0)
             return;
+        
+        if (modelUsage.Count == 0)
+            log.Verbose("Claude Code reported no per-model usage for this run.");
 
         var properties = new Dictionary<string, string>(summaryProperties);
-
-        if (modelUsage.Count > 0)
-        {
-            var usageList = new List<ClaudeCodeModelUsage>(modelUsage.Values);
-            properties[ClaudeCodeServiceMessages.Usage.ModelUsageAttribute] = JsonSerializer.Serialize(usageList);
-        }
+        var usageList = new List<ClaudeCodeModelUsage>(modelUsage.Values);
+        properties[ClaudeCodeServiceMessages.Usage.ModelUsageAttribute] = JsonSerializer.Serialize(usageList);
 
         log.WriteServiceMessage(new ServiceMessage(ClaudeCodeServiceMessages.Usage.Name, properties));
     }