fix: ftintitle can handle a list of ampersanded artists (#6375)

This was inspired by real life events:
https://musicbrainz.org/release/7c4d7a15-6b30-4bef-8b20-af200186fbdb by
the artist Danny L Harle has a a track with a featuring list that
contains "Danny L Harle, Oklou & MNEK".

Before:
```
artist = Danny L Harle, Oklou
track = Crystallise My Tears feat. MNEK
```

After:
```
artist = Danny L Harle
track = Crystallise My Tears feat. Oklou & MNEK
```

beetsplug/ftintitle.py

@@ -71,6 +71,12 @@ def split_on_feat(
     if len(parts) == 2:
         return parts
 
+    # Try comma as separator
+    # (e.g. "Alice, Bob & Charlie" where Bob and Charlie are featuring)
+    if for_artist and "," in artist:
+        comma_parts = artist.split(",", 1)
+        return comma_parts[0].strip(), comma_parts[1].strip()
+
     # Fall back to all tokens including generic separators if no explicit match
     if for_artist:
         regex = re.compile(

docs/changelog.rst

@@ -36,9 +36,11 @@ New features
   3. Comma followed by a space
   4. Slash wrapped by spaces
 
-..
-    Bug fixes
-    ~~~~~~~~~
+Bug fixes
+~~~~~~~~~
+
+- :doc:`plugins/ftintitle`: Fix handling of multiple featured artists with
+  ampersand.
 
 For plugin developers
 ~~~~~~~~~~~~~~~~~~~~~

test/plugins/test_ftintitle.py

@@ -324,6 +324,11 @@ def test_find_feat_part(
         ("Alice feat. Bob", ("Alice", "Bob")),
         ("Alice featuring Bob", ("Alice", "Bob")),
         ("Alice & Bob", ("Alice", "Bob")),
+        ("Alice, Bob & Charlie", ("Alice", "Bob & Charlie")),
+        (
+            "Alice, Bob & Charlie feat. Xavier",
+            ("Alice, Bob & Charlie", "Xavier"),
+        ),
         ("Alice and Bob", ("Alice", "Bob")),
         ("Alice With Bob", ("Alice", "Bob")),
         ("Alice defeat Bob", ("Alice defeat Bob", None)),