Handle empty retrieval tables during article generation #547

Context

Article generation crashes when research produces no snippets because cosine similarity receives an empty 1D array.

Acceptance criteria

• Article generation does not call cosine similarity with an empty snippet matrix.
• Empty retrieval returns no information and preserves normal behavior for populated tables.
• A focused runnable regression check covers the empty-table path.

Out of scope

• Replacing the DuckDuckGo package.
• Changing Wikipedia parsing behavior.