test(cluster): add the self-loop cohesion_score regressions #3558 described

#3558's fix landed but its diff omitted the 3 tests its description claimed;
add them (self-loop ignored, self-loops-only is 0, 0..1 bound holds). Also drop
a changelog line the #3559 cherry-pick injected into the released 0.9.61 section.

CHANGELOG.md

@@ -13,7 +13,6 @@ Full release notes with details on each version: [GitHub Releases](https://githu
 
 ## 0.9.61 (2026-09-12)
 
-- Fix: a dedup collision between a defining page and a page that merely mentions the same entity now correctly keeps the defining page's node for non-Latin filenames (Korean, CJK, Cyrillic, ...) — `_id_prefixes` now slugifies with the same Unicode-aware `normalize_id` every extractor mints an id with, instead of an ASCII-only regex that silently dropped the whole path (#3352, thanks @ayushcodes10).
 - Fix: `graphify.serve` now imports cleanly on Python 3.12 and 3.13. The `chinese` extra pins `jieba-py` from 3.12 onward (0.9.60 mistakenly kept the old `jieba` until 3.14, and its invalid regex escapes are a hard error on 3.12+), and the jieba import now suppresses the tokenizer's `SyntaxWarning` regardless of message or line so it never escalates under `-W error`.
 - Fix: the git hook's rebuild-root guard now rejects a symlink-loop or dangling `.graphify_root` on Python 3.13, whose `Path.resolve()` no longer raises on a loop — the saved root must resolve to a real directory inside the repo before it is adopted.
 

tests/test_cluster.py

@@ -46,6 +46,34 @@ def test_cohesion_score_range():
         score = cohesion_score(G, nodes)
         assert 0.0 <= score <= 1.0
 
+def test_cohesion_score_ignores_self_loops():
+    # #3558: a recursive calls self-loop must not inflate the ratio. Two nodes
+    # with one real edge plus a self-loop is still fully cohesive (1.0), not 2.0.
+    G = nx.Graph()
+    G.add_edge("a", "b")
+    G.add_edge("a", "a")  # recursion self-loop
+    score = cohesion_score(G, ["a", "b"])
+    assert score == 1.0
+
+def test_cohesion_score_self_loops_only_is_zero():
+    # Self-loops alone are not inter-node connectivity.
+    G = nx.Graph()
+    G.add_nodes_from(["a", "b", "c"])
+    for n in ("a", "b", "c"):
+        G.add_edge(n, n)
+    score = cohesion_score(G, ["a", "b", "c"])
+    assert score == 0.0
+
+def test_cohesion_score_range_with_self_loops():
+    # The 0..1 bound holds even when every node carries a self-loop.
+    G = nx.complete_graph(4)
+    G = nx.relabel_nodes(G, {i: str(i) for i in G.nodes})
+    for n in list(G.nodes):
+        G.add_edge(n, n)
+    score = cohesion_score(G, list(G.nodes))
+    assert 0.0 <= score <= 1.0
+    assert score == 1.0
+
 def test_score_all_keys_match_communities():
     G = make_graph()
     communities = cluster(G)