Fix: @async_cached decorator does not respect key_hashers parameter

[proboscis]

---\nMigrated from CyberAgentAILab/pinjected#217\nOriginally created by @proboscis on null\n\n---\n\nThe @async_cached decorator has a bug where the key_hashers parameter is not properly respected. The CustomKeyHasher receives the wrong signature (wrapper function signature instead of original function signature), preventing proper parameter name mapping. See branch fix/async-cached-key-hashers-proper for the fix.