feat: enhance group duty pin functionality to delete old messages

- Updated the `_refresh_pin_for_chat` function to delete the old pinned message after sending a new one, ensuring a cleaner chat experience.
- Modified related unit tests to verify the new deletion behavior, including handling exceptions when the old message cannot be deleted.
- Improved documentation in test cases to reflect the updated functionality and error handling.

duty_teller/handlers/group_duty_pin.py

@@ -103,7 +103,7 @@ async def _schedule_next_update(
 async def _refresh_pin_for_chat(
     context: ContextTypes.DEFAULT_TYPE, chat_id: int
 ) -> Literal["updated", "no_message", "failed"]:
-    """Refresh pinned duty message: send new message, unpin old, pin new, save new message_id.
+    """Refresh pinned duty message: send new message, unpin old, pin new, save new message_id, delete old.
 
     Uses single DB session for message_id, text, next_shift_end (consolidated).
 
@@ -120,6 +120,7 @@ async def _refresh_pin_for_chat(
     if message_id is None:
         logger.info("No pin record for chat_id=%s, skipping update", chat_id)
         return "no_message"
+    old_message_id = message_id
     try:
         msg = await context.bot.send_message(chat_id=chat_id, text=text)
     except (BadRequest, Forbidden) as e:
@@ -140,6 +141,16 @@ async def _refresh_pin_for_chat(
         await _schedule_next_update(context.application, chat_id, next_end)
         return "failed"
     await loop.run_in_executor(None, _sync_save_pin, chat_id, msg.message_id)
+    if old_message_id is not None:
+        try:
+            await context.bot.delete_message(chat_id=chat_id, message_id=old_message_id)
+        except (BadRequest, Forbidden) as e:
+            logger.warning(
+                "Could not delete old pinned message %s in chat_id=%s: %s",
+                old_message_id,
+                chat_id,
+                e,
+            )
     await _schedule_next_update(context.application, chat_id, next_end)
     return "updated"