Add index-based remap edge-case tests if rerank switches to returnDocuments: false

[timon0305]

Problem

rerankResults (src/core/pinecone/rerank.ts) currently reads item.document directly with returnDocuments: true, and its document-mapping edge cases (missing document, duplicate documents, empty and absent data) are covered by the tests added in #224.

If the implementation later switches to an index-based remap (returnDocuments: false, mapping each rerank item's index back into the input results array), that path introduces failure modes the current code cannot hit and that no test exercises:

• An index that is out of range of the input set.
• Duplicate indices in the rerank output.
• An empty rerank result set for a non-empty input.

This issue tracks adding those tests. It is gated on the code change: there is nothing to test while rerank.ts maps by item.document, so no work is needed until (and unless) the index-remap path lands.

Acceptance Criteria

Applies only once rerankResults maps rerank output by index into the input results (returnDocuments: false):

• A test asserts an out-of-range index is handled without throwing (skipped or mapped to an empty result, per the chosen behavior).
• A test asserts duplicate indices are handled predictably.
• A test asserts an empty rerank result set returns empty reranked results with degraded: false.
• Each test asserts the mapped SearchResult fields (id, content, score, metadata, reranked).

[timon0305]

The duplicate and empty-set cases on the live document-mapping path are already covered by #224 (PR #226). This issue is only the index-specific cases (notably out-of-range index), which have no code to exercise until rerank moves to returnDocuments: false. It stays gated on that change.