feat(channel): add ChanClose event #40568

Problem:
Plugins using RPC sockets cannot detect when the peer closes a
`sockconnect()` channel, so reconnect logic has no reliable trigger.

Solution:
Add a `ChanClose` event with channel info before the channel is removed,
matching the existing `ChanOpen`/`ChanInfo` event model.

runtime/doc/autocmd.txt

@@ -397,6 +397,13 @@ BufWriteCmd			Before writing the whole buffer to a file.
 BufWritePost			After writing the whole buffer to a file
 				(should undo the commands for BufWritePre).
 
+							*ChanClose*
+ChanClose			After a channel was closed, before it is removed.
+				This is triggered even when inside an
+				autocommand defined without |autocmd-nested|.
+				Sets these |v:event| keys:
+				    info	as from |nvim_get_chan_info()|
+
 							*ChanInfo*
 ChanInfo			State of channel changed, for instance the
 				client of a RPC channel described itself.

runtime/doc/news.txt

@@ -219,6 +219,7 @@ EVENTS
 
 • |:delmarks| now triggers the |MarkSet| autocommand with line==col==0, same
   as |nvim_buf_del_mark()|
+• |ChanClose| is triggered after a channel is closed, before it is removed.
 • |SessionWritePre| event emits just before |:mksession|.
 • |TextPutPre| and |TextPutPost| are triggered before/after putting text.
 • |TabMoved| is triggered when tabs are reordered.

runtime/doc/options.txt

@@ -2544,6 +2544,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Note: The following events are considered to happen outside of a
 	window context and thus cannot be ignored by 'eventignorewin':
 
+		|ChanClose|,
 		|ChanInfo|,
 		|ChanOpen|,
 		|CmdUndefined|,