fix: read track from item["item"] key to match updated Spotify API response shape

Spotify's playlist_items endpoint now nests the track object under "item"
instead of "track" in each playlist entry. The old key is absent, causing
item.get("track") to return None for every entry and silently drop all tracks.

Also adds stderr debug traces at each component boundary (tool dispatch,
API page summary, per-item accept/skip) to make future silent-empty issues
diagnosable, and switches popularity to .get() since it is absent in the
new response shape.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spotify_client.py

@@ -1,11 +1,16 @@
 import os
 import pathlib
+import sys
 
 from dotenv import load_dotenv
 import spotipy
 from spotipy.oauth2 import SpotifyOAuth
 from spotipy.cache_handler import CacheFileHandler
 
+
+def _dbg(msg: str) -> None:
+    print(f"[spotify-mcp] {msg}", file=sys.stderr, flush=True)
+
 load_dotenv()
 
 _SCOPES = " ".join([
@@ -72,25 +77,41 @@ def list_playlists() -> list[dict]:
 
 
 def get_playlist_tracks(playlist_id: str) -> list[dict]:
+    _dbg(f"get_playlist_tracks called with playlist_id={playlist_id!r}")
     sp = get_client()
     results = []
+    page = 0
     response = sp.playlist_items(playlist_id, additional_types=["track"])
     while response:
-        for item in response["items"]:
-            track = item.get("track")
-            if not track or track.get("type") != "track":
+        items = response.get("items") or []
+        total = response.get("total")
+        _dbg(f"  page {page}: total={total}, items_in_page={len(items)}, next={bool(response.get('next'))}")
+        for idx, item in enumerate(items):
+            # Spotify API returns the track under "item" key (newer API) or "track" key (older)
+            track = item.get("item") or item.get("track")
+            if track is None:
+                _dbg(f"    item[{idx}]: track=None (local/null track), skipping")
                 continue
+            track_type = track.get("type")
+            track_name = track.get("name", "<no-name>")
+            track_uri = track.get("uri", "<no-uri>")
+            if track_type != "track":
+                _dbg(f"    item[{idx}]: SKIPPED type={track_type!r} name={track_name!r} uri={track_uri!r}")
+                continue
+            _dbg(f"    item[{idx}]: OK type={track_type!r} name={track_name!r}")
             results.append({
                 "name": track["name"],
                 "artists": [a["name"] for a in track["artists"]],
                 "album": track["album"]["name"],
                 "duration_ms": track["duration_ms"],
                 "duration": _format_duration(track["duration_ms"]),
-                "popularity": track["popularity"],
+                "popularity": track.get("popularity"),
                 "uri": track["uri"],
                 "added_at": item["added_at"],
             })
+        page += 1
         response = sp.next(response) if response["next"] else None
+    _dbg(f"get_playlist_tracks returning {len(results)} tracks")
     return results