fix(retrieve): preserve rerank scores around empty abstracts

[r266-tech]

Fixes #2330

Summary

• filter empty or whitespace abstracts out of rerank provider requests
• map rerank scores back onto their original candidate indexes while preserving vector fallback scores for empty documents
• add regression coverage for mixed non-empty and empty document batches

Tests

• python3 -m py_compile openviking/retrieve/hierarchical_retriever.py tests/retrieve/test_hierarchical_retriever_rerank.py
• uv run --no-project --with ruff ruff check openviking/retrieve/hierarchical_retriever.py tests/retrieve/test_hierarchical_retriever_rerank.py

uv run pytest tests/retrieve/test_hierarchical_retriever_rerank.py -q could not run in this sparse checkout because editable install requires native OpenViking CLI/engine build artifacts that are not present here.

[github-actions]

PR Reviewer Guide 🔍

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🎫 Ticket compliance analysis ✅ 2330 - Fully compliant Compliant requirements: Filter empty or whitespace abstracts out of rerank provider requests Map rerank scores back onto original candidate indexes while preserving vector fallback scores for empty documents Add regression coverage for mixed non-empty and empty document batches

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🏅 Score: 95

🧪 PR contains tests

🔒 No security concerns identified

✅ No TODO sections

🔀 No multiple PR themes

⚡ No major issues detected

[github-actions]

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