Reject ambiguous library names and empty library-set entries (#33)

* Reject ambiguous library names and empty library-set entries

get_library_id() silently picked the first library matching a
requested name, so two libraries sharing a name would sync whichever
happened to come first in the server's response - not necessarily the
one the operator meant. It now raises ConfigError when a name matches
more than one library.

resolve_libraries() rejects an empty entry produced by an empty
LIBRARY_ID, 'a::b', or a leading/trailing ':' the same way, instead of
silently logging 'library  not found' and moving on.

* Clarify get_library_id's ambiguity docstring and error message

Copilot review: the input can be a name or an ID, and the error said
'Library name' even for an ID collision; use neutral wording. Also
made the docstring's second sentence a complete one.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

dsc/client.py

@@ -166,10 +166,22 @@ class SeafileClient:
         _lg.info("Seafile daemon is stopped")
 
     def get_library_id(self, library) -> Optional[str]:
-        for lib_id, lib_name in self.remote_libraries.items():
-            if library in (lib_id, lib_name):
-                return lib_id
-        return None
+        """
+        Resolve a requested name or ID to the one library ID it names.
+
+        Library IDs are unique, but two libraries can share a name. Silently
+        matching more than one and picking the first would sync whichever
+        happened to come first in the server's response, not necessarily the
+        one requested.
+        """
+        matches = [lib_id for lib_id, lib_name in self.remote_libraries.items()
+                   if library in (lib_id, lib_name)]
+        if len(matches) > 1:
+            raise ConfigError(
+                f"{library!r} matches {len(matches)} libraries on the server; "
+                "use the library ID instead"
+            )
+        return matches[0] if matches else None
 
     def sync_lib(self, lib_id: str, lib_dir: str) -> bool:
         """