fix: prevent in-place dictionary mutation in rerank method #814

[suhaniiz]

📋 PR Checklist

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🔗 Related Issue

Closes #814

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📝 What does this PR do?

This PR resolves an in-place dictionary mutation bug in Reranker.rerank() (app/reranker.py):

• It replaces the direct scoring assignment on referenced objects with a safe shallow copying mechanism (doc.copy()).
• This guarantees that attaching the "rerank_score" key will not alter the caller's original input dataset or introduce unintended side effects across concurrent data processing workflows.

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🗂️ Type of Change

• 🐛 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?

• Tested the affected API endpoints manually
• Ran the backend locally (uvicorn app.main:app --reload)
• Ran the frontend locally (npm run dev inside frontend/)
• Added / updated tests

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📸 Screenshots (if UI change)

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⚠️ Anything to flag for reviewers?

The change relies on Python's built-in dictionary copying mechanics (.copy()), which safely shields the top-level keys from being injected back into the source document dictionaries. It has no negative impact on memory allocations for standard text payloads.

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✅ Self-Review Checklist

• My branch is based on dev, not main
• I have not added any secrets / API keys
• I have not modified main branch or any HuggingFace deployment config
• My code follows the existing style (no unnecessary formatting changes)
• I have updated relevant docs / comments if needed

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