fix: validate edge endpoints exist in graph_builder.py dependency_map

[ivyw960]

GraphBuilder.dependency_map() uses dict.setdefault to register edge endpoints, which silently creates phantom entries when an edge references a node ID that does not exist in the blueprint workflow. Later, refresh_step_statuses tries to look up these phantom parents in step_index and crashes with an uninformative KeyError.

Changes in backend/app/faros/runtime/graph_builder.py:

• Build a node_ids set from the workflow before iterating edges
• For each edge, check that both source and target exist in the node set
• If either is missing, raise ValueError with the offending edge details and node ID(s)
• Drop the setdefault calls since we now validate before appending

Added backend/tests/test_pr_12_graph_builder_edges.py (5 tests): valid edges produce correct upstream/downstream lists, edges with missing source/target/both raise ValueError, and a no-edge blueprint works cleanly. All tests pass.