fix: resolve KeyError when text_key is missing in documents #816

[suhaniiz]

📋 PR Checklist

Thank you for contributing to PDF-Assistant-RAG! 🎉
Please fill out this template before submitting. PRs without it filled in will be closed.

---

🔗 Related Issue

Closes #816

---

📝 What does this PR do?

This PR addresses a runtime crash vulnerability inside Reranker.rerank() (app/reranker.py):

• It replaces direct bracket key access (doc[text_key]) with a safe dictionary retrieval method (doc.get(text_key, "")).
• This change ensures that if malformed, unvalidated, or partial document payloads lacking the target text key are ingested into the pipeline, the system handles it gracefully by falling back to an empty string instead of throwing a critical KeyError that crashes the active request.

---

🗂️ Type of Change

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

---

🧪 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

---

📸 Screenshots (if UI change)

---

⚠️ Anything to flag for reviewers?

Using .get(text_key, "") allows the cross-encoder model to continue scoring other valid document elements smoothly within the batch array without throwing critical thread faults.

---

✅ 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

[vercel]

@suhaniiz is attempting to deploy a commit to the param20h's projects Team on Vercel.

A member of the Team first needs to authorize it.

[suhaniiz]

@param20h ...