fix: packets_connack_sent is not incremented if the ack_flag is non-zero

[zhongwencool]

Fixes: https://emqx.atlassian.net/browse/EMQX-10840

Summary

🤖 Generated by Copilot at 28fd8d3

This pull request refactors the connection management logic from the emqx_cm module to the emqx_channel module, and updates the metrics and tests for the new format of the CONNACK packet. It also removes some unused functions and improves the performance of some channel operations by inlining functions.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update

[zhongwencool]

• ?CHAN_CONN_TAB table store {{ClientId, ChanPid, Mod}, so ets:delete_object do anything here.
• we have already deleted {clientid, ChanPid} in emqx_cm process after ChanPid shutdown here:
  https://github.com/emqx/emqx/blob/master/apps/emqx/src/emqx_cm.erl#L686-L692
  So we don't need this connection_closed/1 function now.

[zmstone]

I remember at least the function was left unchanged previously by @savonarola
but I cannot recall the discussion about why delete_object is still here.

[savonarola]

I added a clarification in 4.4
https://github.com/emqx/emqx/blob/main-v4.4/src/emqx_cm.erl#L173

%% We can't clean CHAN_CONN_TAB because records for dead connections
%% are required for `get_chann_conn_mod/1` function, and `get_chann_conn_mod/1`
%% is used for takeover.

I left the fun empty to avoid attempts to "fix" the things while we have the current mechanism of keeping offline sessions.

[zhongwencool]

-define(CONNACK_PACKET(ReasonCode), #mqtt_packet{
    header = #mqtt_packet_header{type = ?CONNACK},
    variable = #mqtt_packet_connack{
        ack_flags = 0,
        reason_code = ReasonCode
    }
}).

?CONNACK_PACKET/1 lock the ack_flags to 0.