fix(retrieval): allow find without rerank and preserve level-2 rerank scores

[mildred522]

Description

This PR fixes two retrieval issues in the find() / HierarchicalRetriever path:

1. VikingFS.find() incorrectly required rerank_config, even though the documentation defines find() as basic
   semantic/vector retrieval and rerank is optional.
2. HierarchicalRetriever did not preserve rerank scores for level-2 global hits used as initial candidates, so
   direct file hits from global retrieval could bypass rerank ordering in THINKING mode.

These changes keep find() usable without rerank configuration and make rerank behavior more consistent for level-2
retrieval results.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Remove the hard failure in openviking/storage/viking_fs.py so find() works when rerank_config is not
  configured.
• Update openviking/retrieve/hierarchical_retriever.py to rerank level-2 global hits before they are used as initial
  candidates, and avoid sending those same level-2 hits through starting-point rerank logic twice.
• Add regression coverage for both cases:
  • tests/misc/test_vikingfs_find_without_rerank.py
  • tests/retrieve/test_hierarchical_retriever_rerank.py

Testing

I added targeted regression tests for the two fixed behaviors.

Relevant local verification previously completed on this branch:

• py -3.11 -m pytest tests\\misc\\test_vikingfs_find_without_rerank.py tests\\retrieve\ \test_hierarchical_retriever_rerank.py tests\\client\\test_search.py tests\\server\\test_api_search.py -q
• py -3.11 -m pytest tests\\integration\\test_http_integration.py -q
• py -3.11 -m ruff check openviking\\storage\\viking_fs.py openviking\\retrieve\\hierarchical_retriever.py tests\ \misc\\test_vikingfs_find_without_rerank.py tests\\retrieve\\test_hierarchical_retriever_rerank.py

Platform verified:

• Windows
• Linux
• macOS

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

This PR is intentionally scoped to retrieval fixes only.

Behavioral intent:

• find() remains a basic semantic retrieval API and should not require rerank configuration.
• Rerank remains optional, but when rerank is enabled in THINKING mode, level-2 global hits should keep their reranked
  scores consistently.