fix: keep tests free of host MCP stale-runtime probes (#590)

patch.dict(os.environ, clear=True) also drops PYTEST_CURRENT_TEST, which
re-enables live MCP process health checks against operator daemons and
makes merge-gate unit tests environment-dependent. Stub the diagnostic
in conftest so unit tests stay isolated.

tests/conftest.py

@@ -74,6 +74,9 @@ def _reset_mutation_authority(monkeypatch):
     monkeypatch.setattr(mcp_server, "_preflight_capability_baseline_porcelain", None)
     monkeypatch.setattr(mcp_server, "_preflight_resolved_task", None)
     monkeypatch.setattr(mcp_server, "_preflight_reviewer_violation_files", [])
+    # Unit tests must not depend on live host MCP process health. Tests that
+    # use patch.dict(..., clear=True) drop PYTEST_CURRENT_TEST, which would
+    # otherwise re-enable the stale-runtime probe against operator daemons.
     monkeypatch.setattr(
         mcp_server,
         "_check_mcp_runtimes_diagnostics",