.NET: Use IA2AClientFactory to create A2AClient (#5277)

* Refactor A2A extensions to use IA2AClientFactory and add ProtocolSelection sample

- Update A2AAgentCardExtensions to accept IA2AClientFactory instead of A2AClientOptions
- Update A2ACardResolverExtensions to accept IA2AClientFactory
- Update A2AClientExtensions to accept IA2AClientFactory
- Update A2AAgent to use IA2AClientFactory for client creation
- Add A2AAgent_ProtocolSelection sample demonstrating protocol selection
- Add comprehensive unit tests for all changes
- Update README files with new sample reference

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Reorder params: options before loggerFactory in A2A extensions

Move A2AClientOptions parameter before ILoggerFactory in AsAIAgent
and GetAIAgentAsync extension methods to follow the repo convention
of keeping LoggerFactory and CancellationToken as the last parameters.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

dotnet/samples/02-agents/A2A/A2AAgent_ProtocolSelection/A2AAgent_ProtocolSelection.csproj

@@ -0,0 +1,19 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFrameworks>net10.0</TargetFrameworks>
+
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="A2A" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\..\..\src\Microsoft.Agents.AI.A2A\Microsoft.Agents.AI.A2A.csproj" />
+  </ItemGroup>
+
+</Project>

dotnet/samples/02-agents/A2A/A2AAgent_ProtocolSelection/Program.cs

@@ -0,0 +1,36 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample demonstrates how to select the A2A protocol binding (HTTP+JSON vs JSON-RPC) when
+// creating an AIAgent from an A2A agent card using A2AClientOptions.PreferredBindings.
+
+using A2A;
+using Microsoft.Agents.AI;
+
+var a2aAgentHost = Environment.GetEnvironmentVariable("A2A_AGENT_HOST") ?? throw new InvalidOperationException("A2A_AGENT_HOST is not set.");
+
+// Initialize an A2ACardResolver to get an A2A agent card.
+A2ACardResolver agentCardResolver = new(new Uri(a2aAgentHost));
+
+// Get the agent card
+AgentCard agentCard = await agentCardResolver.GetAgentCardAsync();
+
+// Use A2AClientOptions to explicitly select the HTTP+JSON protocol binding.
+// This tells the A2A client factory to prefer the HTTP+JSON interface when the agent card
+// advertises multiple supported interfaces.
+A2AClientOptions options = new()
+{
+    PreferredBindings = [ProtocolBindingNames.HttpJson]
+};
+
+// To prefer JSON-RPC instead, use:
+// A2AClientOptions options = new()
+// {
+//     PreferredBindings = [ProtocolBindingNames.JsonRpc]
+// };
+
+// Create an instance of the AIAgent for an existing A2A agent, using the specified protocol binding.
+AIAgent agent = agentCard.AsAIAgent(options: options);
+
+// Invoke the agent and output the text result.
+AgentResponse response = await agent.RunAsync("Tell me a joke about a pirate.");
+Console.WriteLine(response);

dotnet/samples/02-agents/A2A/A2AAgent_ProtocolSelection/README.md

@@ -0,0 +1,27 @@
+# A2A Agent Protocol Selection
+
+This sample demonstrates how to select the A2A protocol binding when creating an `AIAgent` from an A2A agent card.
+
+A2A agents can expose multiple interfaces with different protocol bindings (e.g., HTTP+JSON, JSON-RPC). By default, `AsAIAgent()` prefers HTTP+JSON with JSON-RPC as a fallback. This sample shows how to use `A2AClientOptions.PreferredBindings` to explicitly control which protocol binding is used.
+
+The sample:
+
+- Connects to an A2A agent server specified in the `A2A_AGENT_HOST` environment variable
+- Configures `A2AClientOptions` to prefer the HTTP+JSON protocol binding
+- Creates an `AIAgent` from the resolved agent card using the specified binding
+- Sends a message to the agent and displays the response
+
+## Prerequisites
+
+Before you begin, ensure you have the following prerequisites:
+
+- .NET 10.0 SDK or later
+- An A2A agent server running and accessible via HTTP
+
+**Note**: These samples need to be run against a valid A2A server. If no A2A server is available, they can be run against the echo-agent that can be spun up locally by following the guidelines at: https://github.com/a2aproject/a2a-dotnet/blob/main/samples/AgentServer/README.md
+
+Set the following environment variable:
+
+```powershell
+$env:A2A_AGENT_HOST="http://localhost:5000"  # Replace with your A2A agent server host
+```

dotnet/samples/02-agents/A2A/README.md

@@ -3,7 +3,7 @@
 These samples demonstrate how to work with Agent-to-Agent (A2A) specific features in the Agent Framework.
 
 For other samples that demonstrate how to use AIAgent instances,
-see the [Getting Started With Agents](../../02-agents/Agents/README.md) samples.
+see the [Getting Started With Agents](../Agents/README.md) samples.
 
 ## Prerequisites
 
@@ -16,6 +16,7 @@ See the README.md for each sample for the prerequisites for that sample.
 |[A2A Agent As Function Tools](./A2AAgent_AsFunctionTools/)|This sample demonstrates how to represent an A2A agent as a set of function tools, where each function tool corresponds to a skill of the A2A agent, and register these function tools with another AI agent so it can leverage the A2A agent's skills.|
 |[A2A Agent Polling For Task Completion](./A2AAgent_PollingForTaskCompletion/)|This sample demonstrates how to poll for long-running task completion using continuation tokens with an A2A agent.|
 |[A2A Agent Stream Reconnection](./A2AAgent_StreamReconnection/)|This sample demonstrates how to reconnect to an A2A agent's streaming response using continuation tokens, allowing recovery from stream interruptions.|
+|[A2A Agent Protocol Selection](./A2AAgent_ProtocolSelection/)|This sample demonstrates how to select the A2A protocol binding (HTTP+JSON vs JSON-RPC) when creating an AIAgent from an A2A agent card using A2AClientOptions.|
 
 ## Running the samples from the console
 

dotnet/samples/02-agents/README.md

@@ -19,3 +19,4 @@ The getting started samples demonstrate the fundamental concepts and functionali
 | [Declarative Agents](./DeclarativeAgents) | Loading and executing AI agents from YAML configuration files |
 | [AG-UI](./AGUI/README.md) | Getting started with AG-UI (Agent UI Protocol) servers and clients |
 | [Dev UI](./DevUI/README.md) | Interactive web interface for testing and debugging AI agents during development |
+| [A2A Agents](./A2A/README.md) | Working with Agent-to-Agent (A2A) specific features |