fix(coderd): sort pinned chats first in GetChats pagination (#24222)

The GetChats SQL query ordered by (updated_at, id) DESC with no
pin_order awareness. A pinned chat with an old updated_at could
land on page 2+ and be invisible in the sidebar's Pinned section.

Add a 4-column ORDER BY: pinned-first flag DESC, negated pin_order
DESC, updated_at DESC, id DESC. The negation trick keeps all sort
columns DESC so the cursor tuple < comparison still works. Update
the after_id cursor clause to match the expanded sort key.

Fix the false handler comment claiming PinChatByID bumps updated_at.

coderd/database/migrations/000466_drop_chat_pagination_index.down.sql

@@ -0,0 +1 @@
+CREATE INDEX idx_chats_owner_updated_id ON chats (owner_id, updated_at DESC, id DESC);

coderd/database/migrations/000466_drop_chat_pagination_index.up.sql

@@ -0,0 +1,5 @@
+-- The GetChats ORDER BY changed from (updated_at, id) DESC to a 4-column
+-- expression sort (pinned-first flag, negated pin_order, updated_at, id).
+-- This index was purpose-built for the old sort and no longer provides
+-- read benefit. The simpler idx_chats_owner covers the owner_id filter.
+DROP INDEX IF EXISTS idx_chats_owner_updated_id;