fix: RuntimeError fallback and stampede protection

[emiliano-go]

Changes

Bug: _store_cache_entry crashes with unhandled RuntimeError (500)

When CachePending.redis is None, the middleware falls back to
_get_pool_state(app).get_async_client(). If no lifespan is registered
this raises RuntimeError, which was not caught by the existing
except (RedisError, OSError) handler, crashing the response with a 500.

Fix: Added RuntimeError to the caught exception types. The error is
logged as a warning and the response is delivered without caching (graceful
degradation).

Bug: Thundering herd / cache stampede

When many concurrent requests arrive just before a cached entry expires, all
of them miss and recompute simultaneously, overwhelming the origin.

Fix: Added stampede_protection: bool = False to cache(). When
enabled and the remaining TTL drops below 10% of the original TTL, a hit
is probabilistically promoted to a miss with probability
1 - (remaining_ttl / threshold). Only a fraction of concurrent requests
recompute, keeping origin load manageable.