Find possible robot positions

You are given a 2D grid where `'.'` represents an open cell and `'#'` represents a wall. A hidden robot is standing on one open cell. The robot's sensor returns four integers: `up`, `down`, `left`, and `right`. - `up`: how many open cells the robot can move upward before hitting a wall or leaving the grid - `down`: how many open cells the robot can move downward before hitting a wall or leaving the grid - `left`: how many open cells the robot can move left before hitting a wall or leaving the grid - `right`: how many open cells the robot can move right before hitting a wall or leaving the grid Write a function to return all open cells `(row, col)` that could be the robot's position given the grid and the four sensor values. Use zero-based indexing. A candidate cell is valid only if the visible open-cell counts in all four directions exactly match the given sensor readings.