Add C++ Backend Support for DFS and Bellman-Ford Algorithms

[prex03]

Changes made

• DFS: Implemented stack-based C++ version for adjacency list and adjacency matrix
• Bellman-Ford: Implemented SPFA-optimized C++ version with negative cycle detection
• Updated Algorithms.hpp, graph.cpp, and algorithms.py to route to C++ backend
• Added comprehensive tests for both algorithms with C++ backend
• New C++ backend tests added for both adjacency list and adjacency matrix representations
• Verified callback functionality, negative weights, and cycle detection