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/lua/vim/_meta/api_keysets.gen.lua

@@ -102,6 +102,7 @@ error('Cannot require a meta file')
 --- |'BufWriteCmd'
 --- |'BufWritePost'
 --- |'BufWritePre'
+--- |'ChanClose'
 --- |'ChanInfo'
 --- |'ChanOpen'
 --- |'CmdUndefined'