fix(retrieval): keep VikingFS.find on quick mode

[ranxi2001]

Description

VikingFS.find() did not pass a retrieval mode to HierarchicalRetriever. When a
reranker is configured, the retriever therefore selected its recursive THINKING path,
even though find is the fast semantic-retrieval API.

This change makes find explicitly request QUICK mode. VikingFS.search() keeps its
existing mode selection, and the public API is unchanged.

Human Involvement

• A human participated in the implementation or review loop
• This PR was generated entirely by AI agents without human participation in the loop

Related Issue

Fixes #4463

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

• pass RetrieverMode.QUICK from VikingFS.find() to the hierarchical retriever;
• assert QUICK mode for text and image find calls; and
• cover the reranker-configured path that previously defaulted to THINKING mode.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Local checks:

• 22 relevant find, hierarchical-retriever, and image search-service tests passed;
• Ruff lint and format checks passed on both changed files;
• Python compile checks and git diff --check passed.

The full server search suite was not runnable because this environment has no
~/.openviking/ov.conf. Editable installation is also unavailable because Cargo and
the generated native openviking/bin/ov artifact are absent. The tests were therefore
run from source using an existing project test environment.

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

Screenshots (if applicable)

Not applicable; this change has no visual output.

Additional Notes

The regression first failed with assert None == 'quick'. A direct probe of the
unmodified retriever with an available reranker also entered two rerank rounds and four
child searches, confirming that the omitted mode selected the recursive path.