fix(stability): stage 6 review round 1 — /ping degrades to pure connectivity when watchdog disabled

Review finding (low, real): the comment claimed /ping degenerates to a pure
connectivity check when TG_WATCHDOG_ENABLED=false, but _wd_last_ok_monotonic is
also stamped by _restart_client (on the disconnect-flap path, which runs before
the watchdog-enabled gate), so with the watchdog off one flap sets age and nothing
ever refreshes it — age grows unbounded past the threshold and /ping returns 503 on
a live connection, spuriously failing the container healthcheck and triggering an
autoheal restart after every flap.

Fix: gate the staleness branch on the watchdog being enabled —
  healthy = connected and (not Config["tg_watchdog_enabled"] or age is None or age < threshold)
so with the watchdog disabled /ping is a pure connectivity check (matching the
intent), and correct the comment to note a flap-restart can stamp age even when the
watchdog is off. New test test_ping_watchdog_disabled_stale_age_still_healthy:
watchdog off + connected + stale age => 200 ok. Adversarially validated — reverting
the gate reds the new test (503) while the watchdog-ON stale-probe test stays green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_stage6_healthcheck.py

@@ -99,6 +99,21 @@ def test_ping_degraded_stale_probe(patch_client):
     assert fake.client.get_me_calls == 0
 
 
+def test_ping_watchdog_disabled_stale_age_still_healthy(patch_client, monkeypatch):
+    # With the watchdog OFF, nothing refreshes age (a disconnect-flap restart can stamp it
+    # once, then it only grows). A stale age must NOT drive /ping to 503 on a live connection
+    # — that would spuriously fail the healthcheck and trigger an autoheal restart. So with the
+    # watchdog disabled, /ping is a pure connectivity check: connected + stale age => healthy.
+    monkeypatch.setitem(api_server.Config, "tg_watchdog_enabled", False)
+    fake, tc = patch_client(age=THRESHOLD + 100, is_connected=True)
+    r = tc.get("/ping")
+    assert r.status_code == 200
+    body = r.json()
+    assert body["status"] == "ok"
+    assert body["connected"] is True
+    assert fake.client.get_me_calls == 0
+
+
 def test_ping_degraded_disconnected(patch_client):
     # Even with a fresh probe age, a disconnected client is unhealthy.
     fake, tc = patch_client(age=1.0, is_connected=False)