test(release): the cut-tunnel cell proves it reached the cut tunnel

Review on #136509 (Greptile): assert_nothing_changed accepts any non-zero
exit, so an unrelated early failure could pass. The cell now asserts the
proxy recorded the EOF on github.com and the output names the failed fetch.

tests/e2e/core/upgrade/network/test_proxy_only_egress.py

@@ -109,3 +109,6 @@ def test_tunnel_cut_to_the_git_host_fails_fast_and_changes_nothing(inst):
         edge.close()
     assert r.secs < 60, f"a cut tunnel took {r.secs:.0f}s to fail\n" + r.report(inst)
     S.assert_nothing_changed(inst, before, r, "tunnel cut to the git host")
+    assert "github.com" in edge.proxy.hosts("eof"), "the update failed before reaching the cut tunnel\n" + r.report(inst)
+    assert "fetch" in r.out.lower() or "github.com" in r.out, (
+        "the failure does not say the git fetch failed\n" + r.report(inst))