Enforce pVACvector pathfinder preconditions

[maksimf]

Summary

• Add a fail-fast precondition check in find_optimal_path to ensure it is only called with a graph that passes check_graph_valid.
• Raise a clear exception when find_optimal_path is invoked with an invalid graph state.
• Update the regression test to validate the expected exception for this out-of-contract invocation.

[susannasiebert]

find_optimal_path should only be called if check_graph_valid returns True for its first return value, as you can see in main method. If anything the case that you are trying to test here should result in a fatal error at the beginning of find_optimal_path as this method should only be called if the graph contains nodes for all desired peptides. So something like:

if sorted(list(graph.nodes())) != sorted(list(seq_dict.keys()))
  raise Exception("Graph doesn't contain nodes for all peptides")

[maksimf]

@susannasiebert Thank you Susanna, is this better?