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/commands/download.py

@@ -67,12 +67,30 @@ class DownloadCommand(BaseCommand):
         is_skip_if_exists: bool,
         password: str | None,
         excludes: list[str],
+    ) -> None:
+        remote = remote_path.split(":", 1)[0] if ":" in remote_path else ""
+        from mdrsclient.client import MdrsClient
+
+        client = MdrsClient.from_remote(remote)
+        client.download(remote_path, local_path, is_recursive, is_skip_if_exists, password, excludes)
+        return
+
+    @classmethod
+    def _download_logic(
+        cls,
+        connection: MDRSConnection,
+        remote_path: str,
+        local_path: str,
+        is_recursive: bool,
+        is_skip_if_exists: bool,
+        password: str | None,
+        excludes: list[str],
     ) -> None:
         # Detect DOI path: "remote:10.xxxx/prefix.ID[/optional/sub/path]"
         path_component = remote_path.split(":", 1)[1] if ":" in remote_path else ""
         if cls._is_doi(path_component):
             remote, doi, subpath = cls._parse_doi_remote_host(remote_path)
-            connection = cls._create_connection(remote)
+
             l_dirname = os.path.realpath(local_path)
             if not os.path.isdir(l_dirname):
                 raise IllegalArgumentException(f"Local directory `{local_path}` not found.")
@@ -126,7 +144,7 @@ class DownloadCommand(BaseCommand):
         r_path = r_path.rstrip("/")
         r_dirname = os.path.dirname(r_path)
         r_basename = os.path.basename(r_path)
-        connection = cls._create_connection(remote)
+
         l_dirname = os.path.realpath(local_path)
         if not os.path.isdir(l_dirname):
             raise IllegalArgumentException(f"Local directory `{local_path}` not found.")