fix(unifi): close the cached client before discarding it (fd leak)

The UniFi poller caches a UnifiClient holding a long-lived httpx.AsyncClient
(a real socket pool). On a login failure or a mid-poll error it set
`_client = None` to force re-auth, but never called close() — orphaning the
connection pool until GC. Under a flapping/unreachable controller that leaks
a file descriptor every failure tick: the same class of bug as the SNMP
engine leak (OSError: [Errno 24] Too many open files).

Add _drop_client() that aclose()s the client (best-effort) before nulling
the cache, and route both discard paths through it. Regression tests cover
both failure paths plus the noop/close-error contracts.

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

plugins/unifi/scheduler.py

@@ -15,6 +15,25 @@ logger = logging.getLogger(__name__)
 _client = None  # UnifiClient instance, initialised on first scrape
 
 
+async def _drop_client() -> None:
+    """Close and discard the cached client.
+
+    The client holds a long-lived ``httpx.AsyncClient`` (a connection pool over
+    real sockets). Nulling the reference without ``aclose()`` orphans that pool
+    until GC, so every failure tick that re-auths would leak file descriptors —
+    the same class of bug that took the app down via SNMP (Errno 24, "Too many
+    open files"). Close first, then drop. Best-effort: a failed close must not
+    break the poll loop.
+    """
+    global _client
+    if _client is not None:
+        try:
+            await _client.close()
+        except Exception:
+            pass
+        _client = None
+
+
 def make_poll_task(app: "Quart") -> ScheduledTask:
     cfg = app.config["PLUGINS"]["unifi"]
     interval = int(cfg.get("poll_interval_seconds", 60))
@@ -51,7 +70,7 @@ async def _do_poll(app: "Quart") -> None:
             await _client.login()
         except Exception as exc:
             logger.warning("UniFi initial login failed: %s", exc)
-            _client = None
+            await _drop_client()
             return
 
     try:
@@ -60,7 +79,7 @@ async def _do_poll(app: "Quart") -> None:
         devices = await _client.get_devices()
     except Exception as exc:
         logger.warning("UniFi poll failed — will retry next tick: %s", exc)
-        _client = None  # force re-auth on next tick
+        await _drop_client()  # close + force re-auth on next tick
         return
 
     scraped_at = datetime.now(timezone.utc)