[TheAlgorithms/Java] Add Optimal Binary Search Tree algorithm

### What would you like to Propose? I want to add an implementation of the Optimal Binary Search Tree algorithm Optimal Binary Search Tree is a classic dynamic programming algorithm that computes the minimum weighted search cost for a set of keys and their access frequencies. https://en.wikipedia.org/wiki/Optimal_binary_search_tree ### Issue details Expected behavior: Given arrays of keys and frequencies, the algorithm should return the minimum possible weighted search cost of a binary search tree containing those keys. Example: - keys: [12, 10, 20, 42, 25, 37] - frequencies: [8, 34, 50, 3, 40, 30] - expected result: 324 Planned/implemented test coverage: - empty input returns `0` - single key returns its frequency - fixed small examples with known expected costs - reference example with expected cost `324` - cross-check against a brute-force solver on small inputs - invalid input handling: - null arrays - mismatched array lengths - duplicate keys - negative frequencies