refactor: extract MdrsClient service layer for library portability

To improve the tool's portability as a Python library, the core logic
has been decoupled from the CLI interface. This allows developers to
programmatically interact with MDRS without relying on CLI-specific
argument parsing or local file-based caches.

- Introduce `MdrsClient` service layer to handle core operations.
- Abstract authentication state using `CacheInterface` and `InMemoryCache`.
- Migrate all CLI commands to utilize `MdrsClient` for execution.
- Separate `Doi` data model from API responses and move to `models/doi.py`.
- Update `README.md` to include Python API usage examples.
- Bump package version to 1.3.17.

mdrsclient/api/doi.py

@@ -5,36 +5,29 @@ from pydantic.dataclasses import dataclass
 
 from mdrsclient.api.base import BaseApi
 from mdrsclient.api.utils import token_check
+from mdrsclient.models.doi import Doi
 
 
 @dataclass(frozen=True)
-class DoiFolderRef:
-    """Nested folder reference returned inside a DOI response.
-
-    The DOI endpoint only returns the folder ``id``; ``laboratory_id`` must be
-    obtained by subsequently calling the folder retrieve endpoint.
-    """
-
+class DoiRetrieveFolderRef:
     id: str
 
 
 @dataclass(frozen=True)
-class DoiResponse:
-    """Response from GET v3/doi/{id}/."""
-
-    # The internal DOI suffix ID returned as a string (e.g. "20260429-001").
+class DoiRetrieveResponse:
     id: str
     doi: str
-    folder: DoiFolderRef
+    folder: DoiRetrieveFolderRef
 
 
 class DoiApi(BaseApi):
     ENTRYPOINT: Final[str] = "v3/doi/"
 
-    def retrieve(self, doi_id: str) -> DoiResponse:
+    def retrieve(self, doi_id: str) -> Doi:
         """Retrieve the folder associated with a DOI suffix ID (GET v3/doi/{id}/)."""
         url = self.ENTRYPOINT + doi_id + "/"
         token_check(self.connection)
         response = self.connection.get(url)
         self._raise_response_error(response)
-        return TypeAdapter(DoiResponse).validate_python(response.json())
+        api_resp = TypeAdapter(DoiRetrieveResponse).validate_python(response.json())
+        return Doi(id=api_resp.id, doi=api_resp.doi, folder_id=api_resp.folder.id)