Move A2A samples from 04-hosting to 02-agents (#5267)

Move the A2A sample projects (A2AAgent_AsFunctionTools and
A2AAgent_PollingForTaskCompletion) from samples/04-hosting/A2A/ to
samples/02-agents/A2A/ to better align with the sample directory
structure. Update solution file and samples README accordingly.

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

dotnet/samples/02-agents/A2A/A2AAgent_PollingForTaskCompletion/Program.cs

@@ -0,0 +1,35 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+// This sample demonstrates how to poll for long-running task completion using continuation tokens with an A2A AI agent.
+
+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();
+
+// Create an instance of the AIAgent for an existing A2A agent specified by the agent card.
+AIAgent agent = agentCard.AsAIAgent();
+
+AgentSession session = await agent.CreateSessionAsync();
+
+// Start the initial run with a long-running task.
+AgentResponse response = await agent.RunAsync("Conduct a comprehensive analysis of quantum computing applications in cryptography, including recent breakthroughs, implementation challenges, and future roadmap. Please include diagrams and visual representations to illustrate complex concepts.", session);
+
+// Poll until the response is complete.
+while (response.ContinuationToken is { } token)
+{
+    // Wait before polling again.
+    await Task.Delay(TimeSpan.FromSeconds(2));
+
+    // Continue with the token.
+    response = await agent.RunAsync(session, options: new AgentRunOptions { ContinuationToken = token });
+}
+
+// Display the result
+Console.WriteLine(response);