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>

tests/test_libraries.py

@@ -0,0 +1,56 @@
+"""A library set must resolve to exactly one library ID per entry, or fail."""
+
+import pytest
+
+from dsc.client import SeafileClient
+from dsc.errors import ConfigError
+from start import resolve_libraries
+
+
+@pytest.fixture
+def client(monkeypatch):
+    c = SeafileClient("seafile.example", "user", "pw", "/dsc")
+    monkeypatch.setattr(type(c), "remote_libraries", {
+        "id-1": "Docs",
+        "id-2": "Docs",
+        "id-3": "Photos",
+    })
+    monkeypatch.setattr(c, "get_local_libraries", lambda: set())
+    return c
+
+
+def test_get_library_id_resolves_a_unique_name(client):
+    assert client.get_library_id("Photos") == "id-3"
+
+
+def test_get_library_id_resolves_by_id(client):
+    assert client.get_library_id("id-1") == "id-1"
+
+
+def test_get_library_id_returns_none_for_no_match(client):
+    assert client.get_library_id("Missing") is None
+
+
+def test_get_library_id_rejects_an_ambiguous_name(client):
+    with pytest.raises(ConfigError, match="Docs.*2 libraries"):
+        client.get_library_id("Docs")
+
+
+def test_resolve_libraries_rejects_an_empty_requested_set(client):
+    with pytest.raises(ConfigError, match="empty entry"):
+        resolve_libraries(client, "", "seafile.example")
+
+
+@pytest.mark.parametrize("requested", ["Photos:", ":Photos", "Photos::id-1"])
+def test_resolve_libraries_rejects_an_empty_entry_between_separators(client, requested):
+    with pytest.raises(ConfigError, match="empty entry"):
+        resolve_libraries(client, requested, "seafile.example")
+
+
+def test_resolve_libraries_rejects_an_ambiguous_name_end_to_end(client):
+    with pytest.raises(ConfigError, match="Docs"):
+        resolve_libraries(client, "Docs", "seafile.example")
+
+
+def test_resolve_libraries_resolves_unambiguous_names(client):
+    assert resolve_libraries(client, "Photos:id-1", "seafile.example") == {"id-3", "id-1"}