Add integration test for GCP forwarding rules sync

[achantavy]

Summary

• add mock global forwarding rule data
• add integration test for sync_gcp_forwarding_rules covering global and regional rules

Testing

• pytest tests/integration/cartography/intel/gcp/test_compute.py -k test_sync_gcp_forwarding_rules -vv (fails: Couldn't connect to localhost:7687)

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https://chatgpt.com/codex/tasks/task_b_68bb2250aef0832fac7a9dbd40f7bbcd