feat(chatd): use last assistant message as push notification summary (#22671)

Instead of the static 'Agent has finished running.' text, extract a
summary from the last assistant message to give users meaningful context
about what the agent accomplished. Falls back to the static text if no
suitable message is found.

Co-authored-by: Kyle Carberry <kyle@carberry.com>

coderd/database/queries/chats.sql

@@ -433,5 +433,18 @@ WHERE id = (
 )
 RETURNING *;
 
+-- name: GetLastChatMessageByRole :one
+SELECT
+    *
+FROM
+    chat_messages
+WHERE
+    chat_id = @chat_id::uuid
+    AND role = @role::text
+ORDER BY
+    created_at DESC, id DESC
+LIMIT
+    1;
+
 -- name: GetChatByIDForUpdate :one
 SELECT * FROM chats WHERE id = @id::uuid FOR UPDATE;