spotify oauth: keep the redirect_uri trailing slash (follow-up to #942)

#942's normalize_spotify_oauth_config trimmed whitespace/quotes (good — those can't be part of a
real credential) but ALSO rstrip("/")'d the redirect_uri. that's unsafe: Spotify matches the
redirect URI EXACTLY against the app's dashboard registration, and a trailing slash is a
legitimate part of a URI. stripping it would silently break anyone who registered '…/callback/'
(we'd send '…/callback' → INVALID_CLIENT: Invalid redirect URI) — trading one failure mode for a
sneakier one the user can't diagnose (SoulSync no longer sends what they typed).

drop the rstrip; keep the whitespace/quote trim. the value is now preserved verbatim apart from
unambiguous paste garbage. flipped the test that asserted the strip to assert the slash is kept
(and that whitespace/quotes around it are still trimmed), + a dedicated regression guard.
the #942 integration test mocks normalize, so it's unaffected. 262 spotify/oauth tests green.

credit: builds on HellRa1SeR's #942.

core/spotify_client.py

@@ -17,9 +17,16 @@ logger = get_logger("spotify_client")
 def normalize_spotify_oauth_config(config: Optional[Dict[str, Any]]) -> Dict[str, Any]:
     """Normalize Spotify OAuth config before building an auth manager.
 
-    Spotify rejects values that include surrounding whitespace, quotes, or
-    newline characters. The settings UI can paste values that carry such
-    formatting, so we trim them before sending them to Spotify.
+    Spotify rejects values that include surrounding whitespace or quotes, and the
+    settings UI can paste values carrying such formatting, so we trim those — they
+    can never be part of a real credential.
+
+    We deliberately do NOT strip a trailing slash from ``redirect_uri``: Spotify
+    matches the redirect URI EXACTLY against the app's dashboard registration, and
+    a trailing slash is a legitimate part of a URI. Stripping it would silently
+    break anyone who registered ``…/callback/`` (we'd send ``…/callback`` →
+    "INVALID_CLIENT: Invalid redirect URI"). So the value is preserved verbatim
+    apart from the unambiguous whitespace/quote garbage (#942 follow-up).
     """
     if not isinstance(config, dict):
         return {}
@@ -28,10 +35,7 @@ def normalize_spotify_oauth_config(config: Optional[Dict[str, Any]]) -> Dict[str
     for key in ("client_id", "client_secret", "redirect_uri"):
         value = config.get(key, "")
         if isinstance(value, str):
-            value = value.strip().strip('"').strip("'")
-            if key == "redirect_uri":
-                value = value.rstrip("/")
-            normalized[key] = value
+            normalized[key] = value.strip().strip('"').strip("'")
         else:
             normalized[key] = value
     return normalized

tests/test_spotify_client.py

@@ -6,19 +6,33 @@ sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
 from core.spotify_client import normalize_spotify_oauth_config
 
 def test_normalization():
-    # Normal case with leading/trailing whitespace and quotes
+    # Whitespace + quotes are stripped (paste garbage); the redirect_uri's
+    # trailing slash is PRESERVED — Spotify matches it exactly against the app
+    # dashboard, so stripping it could break a valid registration (#942 follow-up).
     config = {
-        "client_id": "  client_id   ",
+        "client_id": '  "client_id"   ',
         "client_secret": "  client_secret  ",
-        "redirect_uri": "http://127.0.0.1:8888/callback/"
+        "redirect_uri": "  http://127.0.0.1:8888/callback/  "
     }
     expected = {
         "client_id": "client_id",
         "client_secret": "client_secret",
-        "redirect_uri": "http://127.0.0.1:8888/callback"
+        "redirect_uri": "http://127.0.0.1:8888/callback/"   # slash kept
     }
     assert normalize_spotify_oauth_config(config) == expected
 
+
+def test_trailing_slash_on_redirect_uri_is_preserved():
+    """Regression guard: Spotify requires an EXACT redirect-URI match against the
+    app dashboard, so a trailing slash a user registered must NOT be stripped —
+    stripping it would send '…/callback' and trigger INVALID_CLIENT (#942)."""
+    with_slash = {"client_id": "x", "client_secret": "y",
+                  "redirect_uri": "http://127.0.0.1:8888/callback/"}
+    without_slash = {"client_id": "x", "client_secret": "y",
+                     "redirect_uri": "http://127.0.0.1:8888/callback"}
+    assert normalize_spotify_oauth_config(with_slash)["redirect_uri"] == "http://127.0.0.1:8888/callback/"
+    assert normalize_spotify_oauth_config(without_slash)["redirect_uri"] == "http://127.0.0.1:8888/callback"
+
 def test_empty_values():
     # Empty input values
     config = {