.NET: Durable Agent samples and automated validation for non-Azure Functions

[cgillum]

Motivation and Context

Durable Agents are intended to be available both in Azure Functions and non-Azure Functions contexts. The capability already exists for .NET, but no docs or samples have been created yet.

Description

This PR adds samples showing how to implement the various durable agent scenarios as simple console apps. No Azure Functions is required. Instead, apps use the Durable Task Scheduler directly.

All Azure Functions samples have been converted into their equivalent non-Azure Functions console apps with the exception of the MCP sample, which requires features specific to Azure Functions.

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.