fix: enforce string sorting key to handle mixed metadata types (#797)

[suhaniiz]

📋 PR Checklist

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🔗 Related Issue

Closes #797

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📝 What does this PR do?

Fixes a potential TypeError crash within the graph serialization sanitization pipeline (_convert_sets_for_json).

When processing diverse document chunks, metadata keys like page or chunk_index can occasionally emerge with mixed primitive types (e.g., an integer page identifier 14 alongside an explicit textual page boundary descriptor "appendix_b"). When elements containing these variant types are collected into graph node/edge sets, calling Python's native sorted() without a fallback comparator explicitly throws an unhandled error ('<' not supported between instances of 'str' and 'int').

This PR introduces a uniform string representation key fallback (key=str) within the sorting comprehensions, ensuring string-based sorting ordering evaluations complete predictably across heterogenous document indices without interruption.

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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
• Verified through manual verification that passing collections containing mixed integers and string literals sorts smoothly without comparison errors.

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📸 Screenshots (if UI change)

N/A (Backend logic fix)

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⚠️ Anything to flag for reviewers?

The sorting uses key=str lazily inside the generator wrapper expressions. This gracefully yields an alphanumeric sort without throwing comparison evaluation errors, maintaining stability regardless of arbitrary source document structural discrepancies.

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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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