add per-client close hook

[tyr1k]

This PR adds an optional per-client close hook for deterministic cleanup of connection-scoped resources.

Why:
ldapserver already supports per-connection state via SetData / GetData, but there is currently no explicit callback when a frontend client session ends. In proxy scenarios this makes backend resource cleanup indirect and timing-dependent.

What:

• add Server.OnClientClose
• invoke it once during client.close()
• pass the frontend net.Conn and client-scoped data
• call the hook after request processing/output shutdown and before closing the frontend connection
• recover from hook panic so server shutdown is not blocked

Tests:

• hook fires on LDAP Unbind
• hook fires on abrupt TCP disconnect
• hook receives data set through SetData
• hook runs once per client lifecycle
• panic inside hook does not break Stop()