Unique Paths II - Handling Obstacles

If a cell has an obstacle, the number of paths through that cell is 0. You cannot reach it or pass through it.

Critical: If the starting cell or ending cell has an obstacle, return 0 immediately. No paths exist.

For other cells: if obstacle, set $dp[i][j] = 0$. Otherwise, use the same formula: $dp[i][j] = dp[i-1][j] + dp[i][j-1]$.