Add unit tests for the graph_builder module

[NovaCode37]

modules/graph_builder.py (build_graph) turns scan results into nodes and edges but has no dedicated tests.

Add tests that feed a small sample results dict into build_graph and assert:

• the target node exists
• expected nodes/edges are produced (e.g. an IP from geoip, subdomains from cert_transparency)
• node ids are unique

Good first backend test.

[AbrhamYishak]

can you assign me to this.