refactor: fix broken Edge constructor in RandomGraphGenerator, remove misplaced test file

- RandomGraphGenerator.newEdge() was calling Edge(int, int, int, String)
  which doesn't exist — Edge only has Edge() and Edge(String, String, String)
  constructors. This would cause a compilation error. Fixed to use the
  correct 3-arg String constructor with unique generated identifiers.

- Removed GraphPathExplorerTest.java from src/gvisual/ — test files belong
  in test/gvisual/ where the proper JUnit version already exists. Having a
  standalone test runner in the production source tree bloats the compiled
  artifact and confuses the build.

Author: sauravbhattacharya001