## 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>
## Summary
Change `GetFlattedMetaByKBs` return type from `map[string]interface{}`
to strongly-typed `common.MetaData`.
**Depends on**: #15648 (provides `MetaData`, `MetaValueDocs` types)
### Changes
- `service/metadata.go`: Changed return type, removed type assertions
- `service/metadata_filter.go`: Updated all metadata function signatures
- `service/metadata_filter_test.go` (new): 12 test cases
### Bug fix
`applySingleCondition` used `.([]interface{})` assertions on `[]string`
data, silently breaking operators like `!=`, `contains`, `start with`,
etc.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---------
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
### What problem does this PR solve?
Add cli
LIST DOCUMENTS OF DATASET quoted_string ";"
LIST METADATA OF DATASETS quoted_string ("," quoted_string)* ";"
LIST METADATA SUMMARY OF DATASET quoted_string (DOCUMENTS quoted_string
("," quoted_string)*)? ";"
### Type of change
- [x] Refactoring
