.NET: Simplify A2A client-server sample

[SergeyMenshykh]

Motivation & Context

The A2A client-server sample currently includes several agents and coordination layers, making the basic setup harder to follow. This streamlines it into a focused policy-agent example.

Description & Review Guide

• What are the major changes? Keep one policy agent, use Foundry Responses, simplify the client and server, correct the HTTP request, and update setup guidance.
• What is the impact of these changes? The sample is smaller and quicker to run while retaining agent discovery and both A2A bindings.
• What do you want reviewers to focus on? The simplified end-to-end flow and instructions.

Related Issue

N/A - no matching open issue was found.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) - a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Pull request overview

Simplifies the A2A sample into a focused Foundry-backed policy-agent flow.

Changes:

• Replaces the multi-agent setup with one policy agent.
• Simplifies client discovery, invocation, configuration, and documentation.
• Updates the JSON-RPC request and retains both A2A bindings.

Reviewed changes

Copilot reviewed 12 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Streamlines end-to-end instructions.
A2AServer/README.md	Adds server-specific guidance.
A2AServer/Program.cs	Hosts the Foundry policy agent.
A2AServer/PolicyAgentCard.cs	Defines policy-agent discovery metadata.
A2AServer/Models/InvoiceQuery.cs	Removes invoice mock tooling.
A2AServer/HostAgentFactory.cs	Removes multi-provider agent factory.
A2AServer/A2AServer.http	Updates policy-agent requests.
A2AServer/A2AServer.csproj	Removes unused dependencies.
A2AClient/README.md	Documents the simplified client.
A2AClient/Program.cs	Directly discovers and invokes the agent.
A2AClient/HostClientAgent.cs	Removes coordinating host agent.
A2AClient/A2AClient.csproj	Removes obsolete dependencies.

---

💡 Add a code-review agent skill for context-aware, tailored reviews. Learn more in the docs.

[github-actions]

MAF Automated Review — Iteration 1

Result: Findings reported
Scope: full PR (3 commit(s)): 5c9e0b1b7193, 2e51389ecbcf, 579e619fb06e
Model: gpt-5.6-sol

Overview

The review found 1 verified inline finding(s).

Reviewed the supplied pull-request change set across correctness, security/reliability, architecture, and failure behavior.
1 verified finding remained after source verification (1 medium) across 1 file. Details are attached to the affected lines below.

Affected areas: dotnet/samples/05-end-to-end/A2AClientServer/A2AClient/README.md