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>
