feat(backend/WebApi: implement SSE streaming for agent responses

Added support for streaming agent responses using Server-Sent Events (SSE).
This allows real-time updates to be sent to clients as the agent processes messages. The implementation includes a new service for handling SSE streaming and a mapper for event types based on agent responses.

Modified files (5):
- Program.cs: Updated application startup to include logging and middleware
- AgentEventMapper.cs: Added helper methods for mapping agent updates
- SseStreamingService.cs: Implemented streaming logic for agent responses
- TaskAgentService.cs: Removed obsolete service implementation
- TaskAgent.WebApi.csproj: Updated project dependencies and added logs folder

Author: cristofima

src/backend/services/TaskAgent/src/TaskAgent.WebApi/Constants/AgentConstants.cs

@@ -0,0 +1,62 @@
+namespace TaskAgent.WebApi.Constants;
+
+/// <summary>
+/// Constants for Agent API and SSE event types
+/// </summary>
+public static class AgentConstants
+{
+    /// <summary>
+    /// SSE Content-Type header value
+    /// </summary>
+    public const string SSE_CONTENT_TYPE = "text/event-stream";
+
+    /// <summary>
+    /// SSE Cache-Control header value
+    /// </summary>
+    public const string SSE_CACHE_CONTROL = "no-cache";
+
+    /// <summary>
+    /// SSE Connection header value
+    /// </summary>
+    public const string SSE_CONNECTION = "keep-alive";
+
+    /// <summary>
+    /// User role identifier for message parsing
+    /// </summary>
+    public const string USER_ROLE = "USER";
+
+    /// <summary>
+    /// Custom SSE event type for thread state serialization
+    /// </summary>
+    public const string EVENT_THREAD_STATE = "THREAD_STATE";
+
+    /// <summary>
+    /// SSE event type for run errors
+    /// </summary>
+    public const string EVENT_RUN_ERROR = "RUN_ERROR";
+
+    /// <summary>
+    /// Error type identifier for agent errors
+    /// </summary>
+    public const string ERROR_AGENT_ERROR = "AgentError";
+
+    /// <summary>
+    /// SSE event type for text message content
+    /// </summary>
+    public const string EVENT_TEXT_MESSAGE_CONTENT = "TEXT_MESSAGE_CONTENT";
+
+    /// <summary>
+    /// SSE event type for tool call start
+    /// </summary>
+    public const string EVENT_TOOL_CALL_START = "TOOL_CALL_START";
+
+    /// <summary>
+    /// SSE event type for tool call result
+    /// </summary>
+    public const string EVENT_TOOL_CALL_RESULT = "TOOL_CALL_RESULT";
+
+    /// <summary>
+    /// SSE event type for generic run updates
+    /// </summary>
+    public const string EVENT_RUN_UPDATE = "RUN_UPDATE";
+}

src/backend/services/TaskAgent/src/TaskAgent.WebApi/Constants/ApiRoutes.cs

@@ -5,5 +5,5 @@ namespace TaskAgent.WebApi.Constants;
 /// </summary>
 public static class ApiRoutes
 {
-    public const string CHAT = "api/chat";
+    public const string AGUI = "/agui";
 }

src/backend/services/TaskAgent/src/TaskAgent.WebApi/Controllers/AgentController.cs

@@ -0,0 +1,82 @@
+using System.Text.Json;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.AI;
+using TaskAgent.Application.DTOs.Requests;
+using TaskAgent.WebApi.Constants;
+using TaskAgent.WebApi.Services;
+
+namespace TaskAgent.WebApi.Controllers;
+
+/// <summary>
+/// Custom AG-UI streaming endpoint with serialized state support for conversation continuity.
+/// </summary>
+[ApiController]
+[Route("api/[controller]")]
+public class AgentController : ControllerBase
+{
+    private readonly SseStreamingService _streamingService;
+    private readonly ILogger<AgentController> _logger;
+
+    public AgentController(
+        SseStreamingService streamingService,
+        ILogger<AgentController> logger
+    )
+    {
+        _streamingService = streamingService;
+        _logger = logger;
+    }
+
+    /// <summary>
+    /// Streams agent responses with Server-Sent Events (SSE), including thread state.
+    /// </summary>
+    [HttpPost("chat")]
+    public async Task ChatAsync(
+        [FromBody] AgentRequest request,
+        CancellationToken cancellationToken
+    )
+    {
+        try
+        {
+            // Convert request messages to ChatMessage list
+            List<ChatMessage> messages =
+                request.Messages?.Select(MapToChatMessage).ToList() ?? [];
+
+            // Stream response using the service
+            await _streamingService.StreamToResponseAsync(
+                Response,
+                messages,
+                request.SerializedState,
+                cancellationToken
+            );
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex, "Error processing agent request");
+            await WriteErrorEventAsync(ex.Message, cancellationToken);
+        }
+    }
+
+    private static ChatMessage MapToChatMessage(AgentMessage message)
+    {
+        ChatRole role =
+            message.Role?.ToUpperInvariant() == AgentConstants.USER_ROLE
+                ? ChatRole.User
+                : ChatRole.Assistant;
+
+        return new ChatMessage(role, new List<AIContent> { new TextContent(message.Content ?? "") });
+    }
+
+    private async Task WriteErrorEventAsync(string errorMessage, CancellationToken cancellationToken)
+    {
+        string errorEvent = JsonSerializer.Serialize(
+            new
+            {
+                type = AgentConstants.EVENT_RUN_ERROR,
+                error = AgentConstants.ERROR_AGENT_ERROR,
+                message = errorMessage,
+            }
+        );
+
+        await Response.WriteAsync($"data: {errorEvent}\n\n", cancellationToken);
+    }
+}