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/cache.py

@@ -2,6 +2,7 @@ import dataclasses
 import hashlib
 import json
 import os
+from typing import Protocol, runtime_checkable
 
 from pydantic import TypeAdapter, ValidationError
 from pydantic.dataclasses import dataclass
@@ -16,7 +17,7 @@ from mdrsclient.utils import FileLock
 class CacheData:
     user: User | None = None
     token: Token | None = None
-    laboratories: Laboratories = Laboratories()
+    laboratories: Laboratories = dataclasses.field(default_factory=Laboratories)
     digest: str = ""
 
     def clear(self) -> None:
@@ -43,6 +44,67 @@ class CacheData:
         ).hexdigest()
 
 
+@runtime_checkable
+class CacheInterface(Protocol):
+    @property
+    def token(self) -> Token | None: ...
+    @token.setter
+    def token(self, token: Token) -> None: ...
+    @token.deleter
+    def token(self) -> None: ...
+
+    @property
+    def user(self) -> User | None: ...
+    @user.setter
+    def user(self, user: User) -> None: ...
+    @user.deleter
+    def user(self) -> None: ...
+
+    @property
+    def laboratories(self) -> Laboratories: ...
+    @laboratories.setter
+    def laboratories(self, laboratories: Laboratories) -> None: ...
+
+
+class InMemoryCache:
+    def __init__(self) -> None:
+        self.__data = CacheData()
+
+    @property
+    def token(self) -> Token | None:
+        return self.__data.token
+
+    @token.setter
+    def token(self, token: Token) -> None:
+        self.__data.token = token
+
+    @token.deleter
+    def token(self) -> None:
+        if self.__data.token is not None:
+            self.__data.token = None
+
+    @property
+    def user(self) -> User | None:
+        return self.__data.user
+
+    @user.setter
+    def user(self, user: User) -> None:
+        self.__data.user = user
+
+    @user.deleter
+    def user(self) -> None:
+        if self.__data.user is not None:
+            self.__data.user = None
+
+    @property
+    def laboratories(self) -> Laboratories:
+        return self.__data.laboratories
+
+    @laboratories.setter
+    def laboratories(self, laboratories: Laboratories) -> None:
+        self.__data.laboratories = laboratories
+
+
 class CacheFile:
     __serial: int
     __cache_dir: str