.NET: Add workflow as an agent with observability sample

[TaoChenOSU]

Motivation and Context

We need a sample to show how to enable observability of workflow agents (a workflow used as an AI agent).

Also closing #1814

Description

Add a sample to show how to enable observability for workflow as an agent. The sample enables telemetry at multiple levels:

1. The chat client level
2. The agent level for agents inside the workflow
3. The workflow level (implicit)
4. The workflow agent level

This sample is known to fail after more than one turn. The issue is tracked here: #1613

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.

[Copilot]

Pull Request Overview

This PR adds a new sample demonstrating how to enable OpenTelemetry observability for workflows when used as AI agents. The sample creates a concurrent workflow with French and English language agents, showing telemetry instrumentation at the chat client, agent, workflow, and workflow agent levels. The sample uses an interactive loop with streaming responses and exports traces to both an Aspire dashboard and optionally to Azure Monitor.

Key changes:

• Added WorkflowAsAnAgentObservability sample with OpenTelemetry instrumentation at multiple levels
• Fixed namespace typo in existing WorkflowAsAnAgent sample from "WorkflowAsAnAgentsSample" to "WorkflowAsAnAgentSample"
• Improved prompt text in AgentOpenTelemetry sample to clarify exit instruction

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
WorkflowAsAnAgent/WorkflowHelper.cs (new)	Implements workflow with concurrent French/English agents and telemetry support
WorkflowAsAnAgent/Program.cs (new)	Main program demonstrating OpenTelemetry setup and interactive workflow execution
WorkflowAsAnAgentObservability.csproj (new)	Project file with required dependencies for OpenTelemetry and Azure integration
WorkflowAsAnAgent/WorkflowHelper.cs (existing)	Updated namespace and refactored executor implementation to use route handlers
WorkflowAsAnAgent/Program.cs (existing)	Corrected namespace typo
AgentOpenTelemetry/Program.cs	Enhanced user prompt with exit instruction
agent-framework-dotnet.slnx	Added new sample project to solution