fix: make sub-workflow resolution tolerant of missing references

Extract TryResolveSubWorkflowDefinition that returns null instead of
throwing when a referenced workflow is missing from the library.

Use the tolerant variant in all graph-walking helpers that build
agent indexes, collect agent nodes, and find sub-workflow nodes.
These operations are used for activity UI enrichment and agent
identity resolution — they should degrade gracefully rather than
crash the turn.

The throwing ResolveSubWorkflowDefinition is now only used by
WorkflowRunner.CreateNodeRoute, which is the actual execution
path and correctly fails with a clear error if a reference is
truly missing.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

sidecar/src/Aryx.AgentHost/Services/AgentIdentityResolver.cs

@@ -350,7 +350,12 @@ internal static class AgentIdentityResolver
                 continue;
             }
 
-            WorkflowDefinitionDto subWorkflow = node.ResolveSubWorkflowDefinition(workflowLibrary);
+            WorkflowDefinitionDto? subWorkflow = node.TryResolveSubWorkflowDefinition(workflowLibrary);
+            if (subWorkflow is null)
+            {
+                continue;
+            }
+
             CollectAgentSubworkflowContexts(
                 subWorkflow,
                 workflowLibrary,

sidecar/src/Aryx.AgentHost/Services/WorkflowDefinitionExtensions.cs

@@ -66,6 +66,15 @@ internal static class WorkflowDefinitionExtensions
     public static WorkflowDefinitionDto ResolveSubWorkflowDefinition(
         this WorkflowNodeDto node,
         IReadOnlyDictionary<string, WorkflowDefinitionDto>? workflowLibrary)
+    {
+        return node.TryResolveSubWorkflowDefinition(workflowLibrary)
+            ?? throw new InvalidOperationException(
+                $"Sub-workflow node \"{node.Id}\" references unknown workflow \"{node.Config.WorkflowId}\".");
+    }
+
+    public static WorkflowDefinitionDto? TryResolveSubWorkflowDefinition(
+        this WorkflowNodeDto node,
+        IReadOnlyDictionary<string, WorkflowDefinitionDto>? workflowLibrary)
     {
         ArgumentNullException.ThrowIfNull(node);
 
@@ -81,8 +90,7 @@ internal static class WorkflowDefinitionExtensions
             return workflow;
         }
 
-        throw new InvalidOperationException(
-            $"Sub-workflow node \"{node.Id}\" references unknown workflow \"{node.Config.WorkflowId}\".");
+        return null;
     }
 
     public static bool IsOrchestrationMode(this WorkflowDefinitionDto workflow, string mode)
@@ -190,8 +198,11 @@ internal static class WorkflowDefinitionExtensions
                 continue;
             }
 
-            WorkflowDefinitionDto subWorkflow = node.ResolveSubWorkflowDefinition(workflowLibrary);
-            CollectAgentNodes(subWorkflow, workflowLibrary, agentNodes, visitedWorkflowIds, visitedAnonymousWorkflows);
+            WorkflowDefinitionDto? subWorkflow = node.TryResolveSubWorkflowDefinition(workflowLibrary);
+            if (subWorkflow is not null)
+            {
+                CollectAgentNodes(subWorkflow, workflowLibrary, agentNodes, visitedWorkflowIds, visitedAnonymousWorkflows);
+            }
         }
     }
 
@@ -227,7 +238,12 @@ internal static class WorkflowDefinitionExtensions
                 return node;
             }
 
-            WorkflowDefinitionDto subWorkflow = node.ResolveSubWorkflowDefinition(workflowLibrary);
+            WorkflowDefinitionDto? subWorkflow = node.TryResolveSubWorkflowDefinition(workflowLibrary);
+            if (subWorkflow is null)
+            {
+                continue;
+            }
+
             WorkflowNodeDto? match = FindSubWorkflowNode(
                 subWorkflow,
                 nodeId,

sidecar/tests/Aryx.AgentHost.Tests/AgentIdentityResolverTests.cs

@@ -174,6 +174,23 @@ public sealed class AgentIdentityResolverTests
         Assert.Equal("Inner Review", subworkflow.SubworkflowName);
     }
 
+    [Fact]
+    public void BuildAgentSubworkflowIndex_SkipsUnresolvableSubWorkflowReferences()
+    {
+        WorkflowDefinitionDto workflow = CreateWorkflow(
+            "parent-workflow",
+            [
+                CreateAgent("agent-top-level", "Top Level"),
+                CreateSubworkflow("subworkflow-missing", "Missing Pipeline", workflowId: "nonexistent-workflow"),
+            ],
+            orchestrationMode: "concurrent");
+
+        IReadOnlyDictionary<string, SubworkflowContext> index =
+            AgentIdentityResolver.BuildAgentSubworkflowIndex(workflow);
+
+        Assert.Empty(index);
+    }
+
     private static WorkflowDefinitionDto CreateWorkflow(
         IReadOnlyList<WorkflowNodeDto> nodes,
         string orchestrationMode = "concurrent")