refactor: enforce Literal type for mode field in append_or_prepend function

Enhanced the `mode` parameter to use `Literal` for stricter type checking, ensuring valid values ("prepend", "append") are explicitly enforced.

Author: jkawamoto

src/mcp_bear/__init__.py

@@ -18,7 +18,7 @@
 from functools import partial
 from http import HTTPStatus
 from pathlib import Path
-from typing import cast, AsyncIterator, Final, Any, Mapping
+from typing import cast, AsyncIterator, Final, Any, Mapping, Literal
 from urllib.parse import urlencode, quote
 
 import requests
@@ -253,7 +253,7 @@ async def add_file(
             description="if specified add the file to the corresponding header inside the note", default=None
         ),
         filename: str = Field(description="file name with extension"),
-        mode: str | None = Field(description="adding mode (prepend, append)", default=None),
+        mode: Literal["prepend", "append"] | None = Field(description="adding mode", default=None),
     ) -> None:
         """Append or prepend a file to a note identified by its title or id."""
         if file.startswith("http://") or file.startswith("https://"):