feat(strings): add MoveHashToEnd algorithm (#7313)

* feat(strings): add MoveHashToEnd algorithm with tests

- Add MoveHashToEnd utility class that moves all '#' characters
  to the end of a string while preserving the order of other characters
- Algorithm runs in O(n) time and O(n) space using a two-pass approach:
  first collects non-'#' chars, then fills remaining positions with '#'
- Add null and empty string guards
- Add MoveHashToEndTest with 9 unit tests covering normal, edge,
  and boundary cases (null, empty, all-hash, no-hash, single char)

* docs(strings): add reference URL to MoveHashToEnd Javadoc

* docs: add MoveHashToEnd to DIRECTORY index

* style(strings): add missing newline at EOF in MoveHashToEnd

* test(strings): fix MoveHashToEnd expected output for sample input