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.
