chore: clean up pre-existing ruff violations in tests

Remove unused imports (F401) and strip trailing whitespace (W291/W293) flagged
by ruff in test files. Pre-existing debt unrelated to any single feature; CI runs
pytest but not ruff, so these had accumulated. No behavior change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NGzHtDvJur9U7ysgRKRUTN

tests/test_custom_items.py

@@ -8,7 +8,6 @@ and the error/empty branches.
 
 import asyncio
 
-import pytest
 
 from intervals_mcp_server.tools import custom_items
 
@@ -60,7 +59,7 @@ def test_create_builds_full_payload(monkeypatch):
     rec = _patch(monkeypatch, lambda _k: {"id": 9, "name": "Chart", "type": "FITNESS_CHART"})
     out = _run(
         custom_items.create_custom_item(
-            name="Chart", item_type="FITNESS_CHART", 
+            name="Chart", item_type="FITNESS_CHART",
             description="desc", content={"a": 1}, visibility="PRIVATE",
         )
     )