fix(mcp): scan all message DB shards in get_chat_images (#84)

WeChat rolls a chat's messages over to the next `message_N.db` shard
once the current shard fills up (~100 MB), so any chat older than the
current shard window has its history split across multiple shards. The
other message-query tools — `get_chat_history`, `search_messages`, and
`decode_image` — already iterate every matching shard via the plural
helper `_find_msg_tables_for_user`. Only `get_chat_images` still used
the singular `_find_msg_table_for_user`, which returns the first shard
that contains the user's table.

Effect: every image that lived in a non-first shard was silently
dropped from `get_chat_images`. On a long-lived chat with many images,
the tool would return only the most recent slice and pretend the rest
did not exist.

Fix: switch `get_chat_images` to `_find_msg_tables_for_user`, fetch
`limit` images per shard, merge, sort by `create_time` DESC, and slice
to `limit`. This mirrors how the other tools fan out across shards.

Tests in `tests/test_get_chat_images_multishard.py`:

- `test_collects_images_from_every_shard` — both shards' images appear
  in the output (the regression case)
- `test_global_sort_by_create_time_desc` — newer image from an older
  shard still wins, output is globally sorted (not per-shard concat)
- `test_limit_truncates_globally_across_shards` — limit=3 takes the 3
  newest overall, not "first shard wins"
- `test_no_shards_returns_not_found` — empty shard list path
- `test_all_shards_empty_returns_no_images` — every shard empty path

All 156 tests pass locally (151 baseline + 5 new). Public tool
signature is unchanged; only the internal scanning loop is widened.

mcp_server.py

@@ -2524,14 +2524,28 @@ def get_chat_images(chat_name: str, limit: int = 20) -> str:
     names = get_contact_names()
     display_name = names.get(username, username)
 
-    db_path, table_name = _find_msg_table_for_user(username)
-    if not db_path:
+    # 同 chat 的消息会分散在多个 message_N.db shard 里 (上限 ~100MB/shard 时滚动到下一个);
+    # 单 shard 查找会漏掉其他 shard 的图片。其他工具 (get_chat_history / search_messages /
+    # decode_image) 早已用复数版本 scan 全部 shard, 这里对齐一致。
+    shards = _find_msg_tables_for_user(username)
+    if not shards:
         return f"找不到 {display_name} 的消息记录"
 
-    images = _image_resolver.list_chat_images(db_path, table_name, username, limit)
-    if not images:
+    # 每个 shard 取 limit 张, 合并后按 create_time DESC 全局排序, 取最新 limit 张。
+    # 单 shard 至少够本次返回, 避免一个 shard 凑不出 limit 时其他 shard 没机会贡献。
+    all_images = []
+    for shard in shards:
+        shard_images = _image_resolver.list_chat_images(
+            shard['db_path'], shard['table_name'], username, limit
+        )
+        all_images.extend(shard_images)
+
+    if not all_images:
         return f"{display_name} 无图片消息"
 
+    all_images.sort(key=lambda img: img['create_time'], reverse=True)
+    images = all_images[:limit]
+
     lines = []
     for img in images:
         time_str = datetime.fromtimestamp(img['create_time']).strftime('%Y-%m-%d %H:%M')