[BUGFIX] : Persistent Storage Leak of Knowledge Graphs (GraphRAG)

[hrshjswniii]

🔗 Related Issue

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Closes #539

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📝 What does this PR do?

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This PR implements multi-document chat capabilities, introduces a Recycle Bin trash/restore flow, and resolves a critical persistent storage leak in GraphRAG background cleanup:

1. GraphRAG Persistent Storage Leak Fix (Bug Fix)

• **Background auto-cleanup in [main.py].
• Added a delete_graph try-except block call inside document_cleanup_job() to automatically clean up persisting knowledge graph JSON files of expired/inactive documents (which previously caused disk leaks on the server).
• Unit Test coverage: Added test_cleanup_old_deleted_documents_purges_graph and test_document_cleanup_job_purges_graph in [test_documents.py]to assert correct file removal, database purging, vector cleanup, and graph cleanup.

2. Multi-Document Selection Chat (New Feature)

• RAG Pipeline & Retrieval Pipeline (Backend): Enhanced retrieve(), PDFSearchTool, _candidate_graphs, get_entity_context, and agent execution helper signatures to accept document_ids: List[str] and verify user access for each. Updated tracing decorator signatures to support keyword arguments dynamically.
• WebSocket & SSE handlers (Backend): Updated endpoints to parse document_ids, validate readiness, save multi-doc messages under document_id = None, and cache query results under a sorted comma-joined key.
• Interactive Checkboxes & Indicators (Frontend): Rendered checking inputs in the sidebar list of documents, added selection badge count indicators, updated textarea placeholders, and structured payloads to dispatch document_ids.

3. Recycle Bin / Trash Modal (New Feature)

• Added TrashModal.tsx and custom routing to list, restore, or immediately purge soft-deleted files.

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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 test_cleanup_old_deleted_documents_purges_graph and test_document_cleanup_job_purges_graph in test_documents.py.
  • Added test_retrieve_with_document_ids_list_and_rbac_checks in test_retriever.py.
  • Added test_chat_ask_success_with_document_ids in test_chat.py.
  • Verified all 164 tests pass successfully via 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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⚠️ Anything to flag for reviewers?

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• In main.py, the delete_graph import and invocation is wrapped in a try-except block to prevent database updates or file deletions from failing if a document doesn't have an associated graph yet.
• In test_documents.py, MockDbSessionContext has been updated to commit transactions on context exit so that database purging side effects are accurately tracked by the tests.