refactor(gcp): harden compute engine request validation

[shivv23]

Summary

• add typed GcpApiError for validation/auth/http/serialization failures
• introduce gcp::middleware helpers for required field extraction and authorized JSON request execution
• refactor gcp_compute_engine to remove panic-prone unwrap paths and centralize request execution
• add unit tests for validation and payload parsing paths

Testing

• cargo test build_create_node_payload -- --nocapture (blocked by pre-existing unrelated compile errors in current main branch)