Count Interior Islands After Flooding

You are given an `m x n` binary grid representing land and water. - `1` = land - `0` = water An **island** is a maximal group of land cells connected 4-directionally (up, down, left, right). A **closed island** is an island that does **not** touch the boundary of the grid. Equivalently, you may imagine first flooding away every land cell that is connected to any boundary land cell, and then counting how many connected land components remain. Write a function that returns the number of closed islands in the grid. Clarifications: - Two land cells belong to the same island if they are connected horizontally or vertically. - Any island that touches the first/last row or first/last column should **not** be counted. - You may solve this using DFS, BFS, or Union-Find.