Fix MCP samples: update MCP SDK to 0.8.0-preview.1 and fix README references (#3959)

- Update ModelContextProtocol NuGet package from 0.4.0-preview.3 to 0.8.0-preview.1
- Update System.Net.ServerSentEvents from 10.0.1 to 10.0.3
- Fix OAuth config to use DynamicClientRegistration in Agent_MCP_Server_Auth
- Fix incorrect sample name references in README files

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

dotnet/samples/GettingStarted/ModelContextProtocol/Agent_MCP_Server_Auth/Program.cs

@@ -34,7 +34,10 @@ var transport = new HttpClientTransport(new()
     Name = "Secure Weather Client",
     OAuth = new()
     {
-        ClientId = "ProtectedMcpClient",
+        DynamicClientRegistration = new()
+        {
+            ClientName = "ProtectedMcpClient",
+        },
         RedirectUri = new Uri("http://localhost:1179/callback"),
         AuthorizationRedirectDelegate = HandleAuthorizationUrlAsync,
     }

dotnet/samples/GettingStarted/ModelContextProtocol/Agent_MCP_Server_Auth/README.md

@@ -54,7 +54,7 @@ dotnet run
 
 The protected server will start at `http://localhost:7071`
 
-### Step 3: Run the ModelContextProtocolPluginAuth sample
+### Step 3: Run the Agent_MCP_Server_Auth sample
 
 Finally, run this client: