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.sql.go

@@ -3536,6 +3536,48 @@ func (q *sqlQuerier) GetChatsByOwnerID(ctx context.Context, arg GetChatsByOwnerI
 	return items, nil
 }
 
+const getLastChatMessageByRole = `-- name: GetLastChatMessageByRole :one
+SELECT
+    id, chat_id, model_config_id, created_at, role, content, visibility, input_tokens, output_tokens, total_tokens, reasoning_tokens, cache_creation_tokens, cache_read_tokens, context_limit, compressed
+FROM
+    chat_messages
+WHERE
+    chat_id = $1::uuid
+    AND role = $2::text
+ORDER BY
+    created_at DESC, id DESC
+LIMIT
+    1
+`
+
+type GetLastChatMessageByRoleParams struct {
+	ChatID uuid.UUID `db:"chat_id" json:"chat_id"`
+	Role   string    `db:"role" json:"role"`
+}
+
+func (q *sqlQuerier) GetLastChatMessageByRole(ctx context.Context, arg GetLastChatMessageByRoleParams) (ChatMessage, error) {
+	row := q.db.QueryRowContext(ctx, getLastChatMessageByRole, arg.ChatID, arg.Role)
+	var i ChatMessage
+	err := row.Scan(
+		&i.ID,
+		&i.ChatID,
+		&i.ModelConfigID,
+		&i.CreatedAt,
+		&i.Role,
+		&i.Content,
+		&i.Visibility,
+		&i.InputTokens,
+		&i.OutputTokens,
+		&i.TotalTokens,
+		&i.ReasoningTokens,
+		&i.CacheCreationTokens,
+		&i.CacheReadTokens,
+		&i.ContextLimit,
+		&i.Compressed,
+	)
+	return i, err
+}
+
 const getStaleChats = `-- name: GetStaleChats :many
 SELECT
     id, owner_id, workspace_id, title, status, worker_id, started_at, heartbeat_at, created_at, updated_at, parent_chat_id, root_chat_id, last_model_config_id, archived, last_error