test(relay): assert connector stub never leaks into production paths

CI guard: fails if gateway/ or plugins/ ever imports the test-only stub
connector or defines StubConnector. Matches code leaks (imports / class defs),
not prose mentions, so the transport.py docstring reference to the stub's path
is allowed.

Phase 1 complete. Task 1.6 of the gateway-relay plan.

tests/gateway/relay/test_no_stub_leak.py

@@ -0,0 +1,44 @@
+"""CI guard: the test-only StubConnector must never leak into production paths.
+
+The relay stub connector lives under tests/ and exists only to prove the
+gateway side of the relay without the real (Node) connector. If it ever appears
+under gateway/ or plugins/, that's a production leak — fail loudly.
+"""
+
+from __future__ import annotations
+
+import pathlib
+import re
+
+_REPO_ROOT = pathlib.Path(__file__).resolve().parents[3]
+_FORBIDDEN_DIRS = ("gateway", "plugins")
+# Match actual code leaks (imports / class definitions), not prose mentions in
+# docstrings/comments. A production file that *imports* the stub or *defines*
+# StubConnector is a real leak; a docstring that references the stub's path as
+# documentation is not.
+_LEAK_PATTERNS = (
+    re.compile(r"^\s*(?:from|import)\s+.*stub_connector", re.MULTILINE),
+    re.compile(r"^\s*(?:from|import)\s+.*\bStubConnector\b", re.MULTILINE),
+    re.compile(r"^\s*class\s+StubConnector\b", re.MULTILINE),
+)
+
+
+def test_stub_connector_does_not_leak_into_production_paths():
+    offenders: list[str] = []
+    for top in _FORBIDDEN_DIRS:
+        base = _REPO_ROOT / top
+        if not base.is_dir():
+            continue
+        for path in base.rglob("*.py"):
+            try:
+                text = path.read_text(encoding="utf-8", errors="ignore")
+            except OSError:  # pragma: no cover
+                continue
+            for pat in _LEAK_PATTERNS:
+                if pat.search(text):
+                    offenders.append(
+                        f"{path.relative_to(_REPO_ROOT)} matches {pat.pattern!r}"
+                    )
+    assert not offenders, (
+        "relay test stub leaked into production paths:\n  " + "\n  ".join(offenders)
+    )