Contributes to #9943: Add docstrings and doctests, fix `is_safe` logic, and improve code formatting

[akiels]

Contributes to #9943

This PR improves the count-islands-in-matrix implementation by:

1. Adding docstrings and doctests for all methods (Matrix, is_safe, diffs, and count_islands)

2. Applying code formatting for consistency and pre-commit compliance (e.g., removing trailing whitespace, fixing parentheses).

3. Fixing a logic bug in is_safe:

• Previously, is_safe() could return int values 1 or 0 instead of expected boolean values (True or False).

• This was corrected by adding a condition to self.graph[i][j], which ensures that is_safe() doesn't return the value of a cell (int values 1 or 0) but instead returns True if the cell has value 1 and False otherwise (value 0).

Since this is a small logic fix, I include it in this PR along with the docs, tests, and formatting changes.

All changes pass local testing with doctest and pre-commit hooks.

These changes together improve the clarity, functionality, and test coverage of the implementation.

Thank you for reviewing!