_message_text_cache was a plain dict with no size limit. Every unique
message_id whose text was fetched (for reply-context lookups) stayed in
memory permanently, causing unbounded growth in long-running deployments
with active group chats.
Replace with an OrderedDict and evict the least-recently-used entry
whenever the cache exceeds _FEISHU_MESSAGE_TEXT_CACHE_SIZE (512). Cache
hits call move_to_end() to refresh LRU order. Mirrors the identical
pattern already used by _pending_processing_reactions in the same class.
e8cacb57d5