Feature/multi document selection

[hrshjswniii]

🔗 Related Issue

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Closes #540

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📝 What does this PR do?

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This PR implements multi-document selection chat, enabling users to scope their RAG chat sessions to exactly a selected subset of files in their active workspace.

Key Enhancements:

1. RAG Pipeline Enhancements (Backend):
   • Updated retrieve(), PDFSearchTool, _candidate_graphs, get_entity_context, and agent execution helpers to accept a list of document_ids and verify read authorization checks for each document.
   • Updated the retrieve function tracing decorator signature to accept **kwargs so that capturing parameters like document_ids and workspace doesn't cause TypeErrors.
2. Chat Router & Cache Integration (Backend):
   • Enhanced WebSocket (chat_ws) and SSE (ask_question, ask_question_stream) route handlers to parse, validate readiness/ownership, and save multi-document messages with document_id = None.
   • Cached responses using a sorted comma-joined string representation of the selection list.
3. Sidebar Checkbox Integration (Frontend):
   • Added checkboxes and active selection highlights inside DocumentSidebar.tsx to toggle documents in the selection list.
   • Added a dynamic header badge indicating the number of selected files (e.g., "3 selected").
4. Chat Scoping and Input Fields (Frontend):
   • Updated dashboard state logic in page.tsx to manage and clear selectedDocIds when changing workspaces or switching active documents.
   • Enhanced empty states and textarea placeholders inside ChatPanel.tsx to reflect the multi-file scope (e.g. "Chatting with 3 selected files").
   • Updated chat payloads to forward document_ids instead of document_id when checkboxes are active, and hid the clear/export buttons for multi-document chats to prevent invalid operations.

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🗂️ Type of Change

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• 🐛 Bug fix
• ✨ New feature
• 🔧 Refactor / code cleanup
• 📝 Documentation update
• 🎨 UI / styling change
• ⚙️ CI / tooling / config change
• 🧪 Tests

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🧪 How was this tested?

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• Tested the affected API endpoints manually
• Added / updated tests
  • Added unit test test_retrieve_with_document_ids_list_and_rbac_checks in [test_retriever.py]
  • Added unit test test_chat_ask_success_with_document_ids in [test_chat.py]
  • Updated mock signatures in [test_rag_tools.py]
  • Verified all 162 backend tests pass successfully (backend\.venv\Scripts\python -m pytest backend/tests)
• Ran frontend local typecheck and compilation (npx tsc --noEmit completed with no compilation errors)

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