Redis `clear_pattern` does not prefix patterns that already start with the key prefix

[allen0099]

Problem

AsyncRedisCacheBackend.clear_pattern uses pattern if pattern.startswith(self.key_prefix) else self._make_key(pattern). The base contract says patterns match the logical key, and the Memory backend always prefixes.

Reproduced: with AsyncRedisCacheBackend(key_prefix="cache:") and the default CacheManager (whose own prefix is also cache:), clear_pattern("cache:user:*") is sent unprefixed and clears 0 keys, although cache:cache:user:1 exists.

Proposal

• 0.3.8: always prefix. When that clears nothing and the pattern starts with the prefix, retry the legacy form and emit a DeprecationWarning.
• 0.4.0: remove the fallback (tracked separately).