fix(knowledge): keep rerank passages nonempty

[SiyaoZheng]

Summary

Ensure knowledge rerank providers always receive a non-empty passage without changing the content returned by the search API.

Knowledge-topic content is loaded from SQLite before reranking. A valid topic can have blank content, which previously passed an empty document to the configured rerank provider. Some providers reject empty documents, while others return unstable scores.

This change adds a dedicated rerank_content field with the fallback order content -> summary -> topic_name -> topic id -> generic label. The original content field remains authoritative and unchanged for SearchHit.content.

Area

• Architecture method
• Benchmark
• Use case
• Documentation
• Developer experience
• CI, build, or release

Verification

uv run pytest tests/unit/test_service/test_knowledge_search.py -q
27 passed

make ci
1505 unit tests passed; 78 integration tests passed (6 deselected);
lint, architecture contracts, OpenAPI drift, package build, and wheel smoke test passed

Checklist

• I kept the change scoped to the relevant area.
• I am opening this from a separate branch, not pushing directly to main.
• No documentation change is required because the public response contract is unchanged.
• I added or updated tests when the change affects behavior.
• I did not commit secrets, .env files, dependency folders, or generated output.
• Active relative links in Markdown files resolve.

Notes for Reviewers

The tests cover SQLite content, metadata fallbacks, the all-blank case, and preservation of the API-facing content value.

By submitting this pull request, I agree that my contribution is licensed under the Apache License 2.0.