## Summary
Add `EnrichChunksWithDocMetadata` as a method on `MetadataService` that
attaches document metadata to retrieval chunks in-place. Equivalent to
Python's `enrich_chunks_with_document_metadata()` from
`api/utils/reference_metadata_utils.py`.
### Usage
```go
metadataSvc.EnrichChunksWithDocMetadata(chunks, tenantID, metadataFields)
```
### Changes
- **`service/metadata.go`**: Added `EnrichChunksWithDocMetadata` method
- **`service/enrich_metadata_test.go`** (new): 7 test cases
### Algorithm
1. Collect unique `(kb_id, doc_id)` pairs from chunks
2. Fetch metadata from ES via `SearchMetadata(kbID, tenantID, docIDs)`
3. Attach `document_metadata` field to each matching chunk
4. Optionally filter to specified `metadataFields`
### Testing
All 7 tests pass:
```
=== RUN TestEnrichChunksWithDocMetadata_NoChunks --- PASS
=== RUN TestEnrichChunksWithDocMetadata_EmptyChunks --- PASS
=== RUN TestEnrichChunksWithDocMetadata_EmptyDocID --- PASS
=== RUN TestEnrichChunksWithDocMetadata_DuplicateDocIDs --- PASS
=== RUN TestEnrichChunksWithDocMetadata_MultipleKBs --- PASS
=== RUN TestEnrichChunksWithDocMetadata_WithMetadataFields --- PASS
=== RUN TestEnrichChunksWithDocMetadata_MixedFields --- PASS
```
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
