Fix GetExecutorName to handle agent names with underscores

Split on last underscore instead of first, and validate that the
suffix is a 32-char hex string (sanitized GUID) before stripping it.
This prevents truncation of agent names like 'my_agent' when the
executor ID is 'my_agent_<guid>'.

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

dotnet/src/Microsoft.Agents.AI.DurableTask/Workflows/WorkflowNamingHelper.cs

@@ -63,8 +63,38 @@ internal static class WorkflowNamingHelper
     {
         ArgumentException.ThrowIfNullOrEmpty(executorId);
 
-        int separatorIndex = executorId.IndexOf(ExecutorIdSuffixSeparator);
-        return separatorIndex > 0 ? executorId[..separatorIndex] : executorId;
+        int separatorIndex = executorId.LastIndexOf(ExecutorIdSuffixSeparator);
+        if (separatorIndex > 0)
+        {
+            ReadOnlySpan<char> suffix = executorId.AsSpan(separatorIndex + 1);
+            if (IsGuidSuffix(suffix))
+            {
+                return executorId[..separatorIndex];
+            }
+        }
+
+        return executorId;
+    }
+
+    /// <summary>
+    /// Checks whether the given span looks like a sanitized GUID (32 hex characters).
+    /// </summary>
+    private static bool IsGuidSuffix(ReadOnlySpan<char> value)
+    {
+        if (value.Length != 32)
+        {
+            return false;
+        }
+
+        foreach (char c in value)
+        {
+            if (!char.IsAsciiHexDigit(c))
+            {
+                return false;
+            }
+        }
+
+        return true;
     }
 
     private static bool TryGetWorkflowName(string? orchestrationFunctionName, [NotNullWhen(true)] out string? workflowName)

dotnet/tests/Microsoft.Agents.AI.DurableTask.UnitTests/Workflows/WorkflowNamingHelperTests.cs

@@ -64,6 +64,22 @@ public sealed class WorkflowNamingHelperTests
         Assert.Equal("Physicist", result);
     }
 
+    [Fact]
+    public void GetExecutorName_NameWithUnderscoresAndGuidSuffix_ReturnsFullName()
+    {
+        string result = WorkflowNamingHelper.GetExecutorName("my_agent_8884e71021334ce49517fa2b17b1695b");
+
+        Assert.Equal("my_agent", result);
+    }
+
+    [Fact]
+    public void GetExecutorName_NameWithUnderscoreButNoGuidSuffix_ReturnsSameName()
+    {
+        string result = WorkflowNamingHelper.GetExecutorName("my_custom_executor");
+
+        Assert.Equal("my_custom_executor", result);
+    }
+
     [Theory]
     [InlineData(null)]
     [InlineData("")]