Refactor public network API handling and enhance connectivity checks

- Updated the public network handling to resolve the base URL dynamically based on Keystone catalog.
- Simplified the public network pool function to directly return the compute region.
- Enhanced error messages in the doctor command for better clarity on public network status.
- Introduced new functions for managing security groups and ensuring visibility of public ports in Neutron.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_public_net_sg.py

@@ -0,0 +1,21 @@
+"""SG for direct public IP uses compute-pool Neutron (openstacksdk)."""
+
+from unittest.mock import MagicMock
+
+from gpu_rent.public_net import SecurityGroupRef, ensure_public_network_security_group
+
+
+def test_public_network_sg_uses_sdk(monkeypatch):
+    main_conn = MagicMock()
+
+    def fake_ensure_sg(conn, cidr, log):
+        assert conn is main_conn
+        assert cidr == "0.0.0.0/0"
+        return MagicMock(id="sg-ru6")
+
+    monkeypatch.setattr("gpu_rent.cloud.ensure_security_group", fake_ensure_sg)
+
+    sg = ensure_public_network_security_group(
+        main_conn, "0.0.0.0/0", "ru-6", lambda _m: None
+    )
+    assert sg == SecurityGroupRef(id="sg-ru6")