fix(security): enforce API_SERVER_KEY for non-loopback binding

Add is_network_accessible() helper using Python's ipaddress module to
robustly classify bind addresses (IPv4/IPv6 loopback, wildcards,
mapped addresses, hostname resolution with DNS-failure-fails-closed).

The API server connect() now refuses to start when the bind address is
network-accessible and no API_SERVER_KEY is set, preventing RCE from
other machines on the network.

Co-authored-by: entropidelic <entropidelic@users.noreply.github.com>

tests/gateway/test_api_server_bind_guard.py

@@ -0,0 +1,132 @@
+"""Tests for the API server bind-address startup guard.
+
+Validates that is_network_accessible() correctly classifies addresses and
+that connect() refuses to start on non-loopback without API_SERVER_KEY.
+"""
+
+import socket
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from gateway.config import PlatformConfig
+from gateway.platforms.api_server import APIServerAdapter
+from gateway.platforms.base import is_network_accessible
+
+
+# ---------------------------------------------------------------------------
+# Unit tests: is_network_accessible()
+# ---------------------------------------------------------------------------
+
+
+class TestIsNetworkAccessible:
+    """Direct tests for the address classification helper."""
+
+    # -- Loopback (safe, should return False) --
+
+    def test_ipv4_loopback(self):
+        assert is_network_accessible("127.0.0.1") is False
+
+    def test_ipv6_loopback(self):
+        assert is_network_accessible("::1") is False
+
+    def test_ipv4_mapped_loopback(self):
+        # ::ffff:127.0.0.1 — Python's is_loopback returns False for mapped
+        # addresses; the helper must unwrap and check ipv4_mapped.
+        assert is_network_accessible("::ffff:127.0.0.1") is False
+
+    # -- Network-accessible (should return True) --
+
+    def test_ipv4_wildcard(self):
+        assert is_network_accessible("0.0.0.0") is True
+
+    def test_ipv6_wildcard(self):
+        # This is the bypass vector that the string-based check missed.
+        assert is_network_accessible("::") is True
+
+    def test_ipv4_mapped_unspecified(self):
+        assert is_network_accessible("::ffff:0.0.0.0") is True
+
+    def test_private_ipv4(self):
+        assert is_network_accessible("10.0.0.1") is True
+
+    def test_private_ipv4_class_c(self):
+        assert is_network_accessible("192.168.1.1") is True
+
+    def test_public_ipv4(self):
+        assert is_network_accessible("8.8.8.8") is True
+
+    # -- Hostname resolution --
+
+    def test_localhost_resolves_to_loopback(self):
+        loopback_result = [
+            (socket.AF_INET, socket.SOCK_STREAM, 0, "", ("127.0.0.1", 0)),
+        ]
+        with patch("gateway.platforms.base._socket.getaddrinfo", return_value=loopback_result):
+            assert is_network_accessible("localhost") is False
+
+    def test_hostname_resolving_to_non_loopback(self):
+        non_loopback_result = [
+            (socket.AF_INET, socket.SOCK_STREAM, 0, "", ("10.0.0.1", 0)),
+        ]
+        with patch("gateway.platforms.base._socket.getaddrinfo", return_value=non_loopback_result):
+            assert is_network_accessible("my-server.local") is True
+
+    def test_hostname_mixed_resolution(self):
+        """If a hostname resolves to both loopback and non-loopback, it's
+        network-accessible (any non-loopback address is enough)."""
+        mixed_result = [
+            (socket.AF_INET, socket.SOCK_STREAM, 0, "", ("127.0.0.1", 0)),
+            (socket.AF_INET, socket.SOCK_STREAM, 0, "", ("10.0.0.1", 0)),
+        ]
+        with patch("gateway.platforms.base._socket.getaddrinfo", return_value=mixed_result):
+            assert is_network_accessible("dual-host.local") is True
+
+    def test_dns_failure_fails_closed(self):
+        """Unresolvable hostnames should require an API key (fail closed)."""
+        with patch(
+            "gateway.platforms.base._socket.getaddrinfo",
+            side_effect=socket.gaierror("Name resolution failed"),
+        ):
+            assert is_network_accessible("nonexistent.invalid") is True
+
+
+# ---------------------------------------------------------------------------
+# Integration tests: connect() startup guard
+# ---------------------------------------------------------------------------
+
+
+class TestConnectBindGuard:
+    """Verify that connect() refuses dangerous configurations."""
+
+    @pytest.mark.asyncio
+    async def test_refuses_ipv4_wildcard_without_key(self):
+        adapter = APIServerAdapter(PlatformConfig(enabled=True, extra={"host": "0.0.0.0"}))
+        result = await adapter.connect()
+        assert result is False
+
+    @pytest.mark.asyncio
+    async def test_refuses_ipv6_wildcard_without_key(self):
+        adapter = APIServerAdapter(PlatformConfig(enabled=True, extra={"host": "::"}))
+        result = await adapter.connect()
+        assert result is False
+
+    def test_allows_loopback_without_key(self):
+        """Loopback with no key should pass the guard."""
+        adapter = APIServerAdapter(PlatformConfig(enabled=True, extra={"host": "127.0.0.1"}))
+        assert adapter._api_key == ""
+        # The guard condition: is_network_accessible(host) AND NOT api_key
+        # For loopback, is_network_accessible is False so the guard does not block.
+        assert is_network_accessible(adapter._host) is False
+
+    @pytest.mark.asyncio
+    async def test_allows_wildcard_with_key(self):
+        """Non-loopback with a key should pass the guard."""
+        adapter = APIServerAdapter(
+            PlatformConfig(enabled=True, extra={"host": "0.0.0.0", "key": "sk-test"})
+        )
+        # The guard checks: is_network_accessible(host) AND NOT api_key
+        # With a key set, the guard should not block.
+        assert adapter._api_key == "sk-test"
+        assert is_network_accessible("0.0.0.0") is True
+        # Combined: the guard condition is False (key is set), so it passes