.NET: Add Foundry Agents Tool Sample - OpenAPI Tools

[rogerbarreto]

Motivation and Context

• Python: Integrate directly with Foundry Tools from agents using any chat clients #3674

Description

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

Adds a new .NET GettingStarted sample under FoundryAgents demonstrating how to configure and run Azure Foundry Agents with an OpenAPI tool (calling a public REST API via an embedded OpenAPI spec).

Changes:

• Introduces a new Step19 sample showing OpenAPI tool usage against the REST Countries API.
• Documents prerequisites, environment variables, and expected behavior for running the sample.
• Adds a dedicated project file referencing the needed Azure/Agent Framework packages.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step19_OpenAPITools/README.md	New sample documentation for OpenAPI tools usage and how to run the sample.
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step19_OpenAPITools/Program.cs	Implements agent creation + OpenAPI tool configuration and executes a sample query.
dotnet/samples/GettingStarted/FoundryAgents/FoundryAgents_Step19_OpenAPITools/FoundryAgents_Step19_OpenAPITools.csproj	Adds the new sample project and package/project references.