.NET: Add error handling to workflow samples with agents

[Copilot]

Motivation and Context

Workflow samples with agents failed silently when executors encountered errors. No error messages were displayed, no exceptions were thrown, making failures invisible to developers.

Description

Added error event handling to 13 workflow samples using agents. Two patterns applied:

For workflows using InProcessExecution.StreamAsync() + WatchStreamAsync():

await foreach (WorkflowEvent evt in run.WatchStreamAsync())
{
    switch (evt)
    {
        case ExecutorFailedEvent failureEvent:
            Console.ForegroundColor = ConsoleColor.Red;
            Console.WriteLine($"Executor failed [{failureEvent.ExecutorId}]: {failureEvent.Data?.Message ?? "Unknown error"}");
            Console.ResetColor();
            break;

        case WorkflowErrorEvent errorEvent:
            Console.ForegroundColor = ConsoleColor.Red;
            Console.WriteLine($"Workflow error: {errorEvent.Exception?.Message ?? "Unknown error"}");
            Console.ResetColor();
            throw errorEvent.Exception ?? new InvalidOperationException("Workflow encountered an error.");
    }
}

For workflows as agents using RunStreamingAsync():

try
{
    await foreach (var update in workflowAgent.RunStreamingAsync(input))
    {
        // ... process updates
    }
}
catch (Exception ex)
{
    Console.ForegroundColor = ConsoleColor.Red;
    Console.WriteLine($"\nWorkflow error: {ex.Message}");
    Console.ResetColor();
    throw;
}

Updated samples:

• Foundational: 03_AgentsInWorkflows, 04_AgentWorkflowPatterns, 05_MultiModelService, 07_MixedWorkflowAgentsAndExecutors, 08_WriterCriticWorkflow
• Agents: CustomAgentExecutors, FoundryAgent, GroupChatToolApproval, WorkflowAsAnAgent
• ConditionalEdges: 01_EdgeCondition, 02_SwitchCase, 03_MultiSelection
• Concurrent: Concurrent

Errors now display in red and cause proper exit codes instead of silent failures.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

Original prompt

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This section details on the original issue you should resolve

<issue_title>.NET: [Bug]: Workflow Foundational Sample 03_AgentsInWorkflow will fail silently if the agent fails</issue_title>
<issue_description>Errors are not being properly checked in the AgentsInWorkflow sample (and likely other samples).</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes .NET: [Bug]: Workflow Foundational Sample 03_AgentsInWorkflow will fail silently if the agent fails #3991

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