[Refactor-Unified-Server-Chat]: Server Chat Response Simplifying

[Castro19]

💡 User Story

As a developer, I want to consolidate the chat message flow into a single backend endpoint so that the frontend remains simple, consistent, and synced with the server-side source of truth.

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📖 Description

We are refactoring the current message lifecycle to reduce complexity and improve maintainability. Currently, the frontend handles multiple responsibilities: generating IDs, managing log creation/updating, and calling multiple backend endpoints. This leads to redundant state management, increased surface area for bugs, and difficult onboarding for new contributors.

The new plan involves:

• Creating a single backend endpoint (/chat-message) that handles:
  • Generating IDs and timestamps
  • Checking for new vs. existing chats
  • Creating or updating the chat log
  • Streaming the assistant response
  • Sending a final JSON payload to the frontend
• Shifting all orchestration responsibility to the backend
• Simplifying the frontend to call only one Redux thunk
• Ensuring the Redux state is updated from the server's authoritative response

This change improves reliability, error handling, and developer clarity.

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✅ Acceptance Criteria

• ✅ A single endpoint handles message submission, log creation/update, and streaming
• ✅ Backend generates all IDs and timestamps
• ✅ Final server response includes updated log, messages, and metadata
• ✅ Frontend uses one Redux thunk (sendMessageThunk) to send message and stream assistant response
• ✅ Redux state is updated solely from server response
• ✅ Errors during any part of the process are properly handled and reported to the frontend

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📌 Technical Details

• Server will verify if a chat ID is new or existing and take appropriate action
• The final JSON response should include chatId, messages, timestamp, and optional title
• Redux logSlice and messageSlice remain separate but are updated together from server response
• Frontend no longer creates UUIDs or timestamps
• Streaming remains functional, with partial updates handled via SSE or chunked fetch

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🖼️ UI/UX Mockups (If applicable)

N/A – No UI changes, backend-focused refactor

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📎 Related Issues or Dependencies

• Related to #XX (Streaming improvements)
• Related to #XX (Log structure updates)

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🚀 Priority & Story Points

• Priority: High
• Story Points: 5