refactor: convert script to automtu package with CI workflow

https://chatgpt.com/share/697112b2-0410-800f-93ff-9372b603d43f

tests/unit/test_pmtu.py

@@ -0,0 +1,36 @@
+import unittest
+from unittest.mock import patch
+
+import automtu.pmtu as pmtu
+
+
+class TestPmtu(unittest.TestCase):
+    def test_probe_pmtu_binary_search_and_hdr_addition(self) -> None:
+        # Mock _is_ipv6 -> IPv4, so hdr = 28.
+        # Mock _ping_ok so that payload <= 1400 works, >1400 fails.
+        def fake_ping_ok(payload: int, target: str, timeout_s: float) -> bool:
+            return payload <= 1400
+
+        with (
+            patch("automtu.pmtu._is_ipv6", return_value=False),
+            patch("automtu.pmtu._ping_ok", side_effect=fake_ping_ok),
+        ):
+            # lo=1200 works, hi=1472 partially works -> best = 1400 -> mtu = 1400+28 = 1428
+            mtu = pmtu.probe_pmtu(
+                "1.1.1.1", lo_payload=1200, hi_payload=1472, timeout=1.0
+            )
+            self.assertEqual(mtu, 1428)
+
+    def test_probe_pmtu_returns_none_if_even_floor_fails(self) -> None:
+        with (
+            patch("automtu.pmtu._is_ipv6", return_value=False),
+            patch("automtu.pmtu._ping_ok", return_value=False),
+        ):
+            mtu = pmtu.probe_pmtu(
+                "1.1.1.1", lo_payload=1200, hi_payload=1472, timeout=1.0
+            )
+            self.assertIsNone(mtu)
+
+
+if __name__ == "__main__":
+    unittest.main(verbosity=2)