fix(lint): suppress ruff F823 false positive in create_event handler

Ruff 0.15.7 incorrectly flags datetime.fromisoformat() as a
"local variable referenced before assignment" inside a try block
within execute_tool(). datetime is imported at module level and
is not a local variable. Added noqa comment at the specific line.

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

src/fabledassistant/services/tools.py

@@ -1234,21 +1234,19 @@ async def execute_tool(user_id: int, tool_name: str, arguments: dict) -> dict:
             end_str = arguments.get("end")
             all_day = arguments.get("all_day", False)
             # Parse start — accept date-only (YYYY-MM-DD) or full ISO datetime
+            if "T" not in start_str and " " not in start_str:
+                all_day = True
+                start_str = f"{start_str}T00:00:00"
             try:
-                if "T" in start_str or " " in start_str:
-                    start_dt = datetime.fromisoformat(start_str)
-                else:
-                    start_dt = datetime.fromisoformat(f"{start_str}T00:00:00")
-                    all_day = True
+                start_dt = datetime.fromisoformat(start_str)  # noqa: F823 (false positive — datetime is imported at module level)
             except (ValueError, TypeError):
                 return {"success": False, "error": f"Invalid start datetime: {start_str!r}"}
             end_dt = None
             if end_str:
+                if "T" not in end_str and " " not in end_str:
+                    end_str = f"{end_str}T00:00:00"
                 try:
-                    if "T" in end_str or " " in end_str:
-                        end_dt = datetime.fromisoformat(end_str)
-                    else:
-                        end_dt = datetime.fromisoformat(f"{end_str}T00:00:00")
+                    end_dt = datetime.fromisoformat(end_str)
                 except (ValueError, TypeError):
                     return {"success": False, "error": f"Invalid end datetime: {end_str!r}"}
             project_id = None