### What problem does this PR solve?
- added the new dataset search route and handler
- reused the existing shared SearchDatasets service by adapting
single-dataset requests into dataset_ids=[dataset_id]
- aligned handler error responses with Python behavior for argument/data
errors
- aligned key service error messages such as invalid search_id and mixed
embedding models
- added focused handler and service tests for request mapping and error
behavior
### Tests:
`/usr/local/go/bin/go test ./internal/service -run
'TestSearchDatasetRequestToSearchDatasetsRequest|TestDatasetServiceSearchDatasets'`
`/usr/local/go/bin/go test ./internal/handler -run
'TestDatasetsHandlerSearchDataset'`
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