.NET: Add error checking to workflow samples

[Copilot]

Motivation and Context

Workflow samples silently swallow WorkflowErrorEvent and ExecutorFailedEvent events — if an agent or executor fails mid-workflow, the sample loop exits cleanly with no indication of what went wrong. Fixes #[issue].

Description

Added WorkflowErrorEvent and ExecutorFailedEvent handling to all 20 workflow samples in dotnet/samples/03-workflows/ that lacked it. This covers both streaming (WatchStreamAsync) and non-streaming (NewEvents) loops, across switch and if/else if patterns.

Error output pattern applied consistently:

else if (evt is WorkflowErrorEvent workflowError)
{
    Console.ForegroundColor = ConsoleColor.Red;
    Console.Error.WriteLine(workflowError.Exception?.ToString() ?? "Unknown workflow error occurred.");
    Console.ResetColor();
}
else if (evt is ExecutorFailedEvent executorFailed)
{
    Console.ForegroundColor = ConsoleColor.Red;
    Console.Error.WriteLine($"Executor '{executorFailed.ExecutorId}' failed with {(executorFailed.Data == null ? "unknown error" : $"exception {executorFailed.Data}")}.");
    Console.ResetColor();
}

Consistent with the existing pattern already used in Concurrent/Concurrent and Agents/CustomAgentExecutors.

Files updated (20): _StartHere/01–03, 05–07, Agents/FoundryAgent, Agents/GroupChatToolApproval, Checkpoint/CheckpointAndRehydrate, Checkpoint/CheckpointAndResume, Checkpoint/CheckpointWithHumanInTheLoop, Concurrent/MapReduce, ConditionalEdges/01–03, HumanInTheLoop/HumanInTheLoopBasic, Loop, Observability/ApplicationInsights, Observability/AspireDashboard, SharedStates.

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.