Re: [vim/vim] runtime(doc): a server request with a String id cannot be answered (PR #21114)

h-east left a comment (vim/vim#21114)

@chrisbra 
Before documenting this limitation, is it worth removing instead? The
restriction looks narrower than the current check makes it.

In `ch_sendexpr()` the "id" is only ever used to match a reply: it is handed
to `channel_set_req_callback()` in `send_common()`, and only when a callback
was given. For `ch_evalexpr()`, and for `ch_sendexpr()` with a callback, Vim
assigns `++channel->ch_last_msg_id` and overwrites whatever the caller put in
"id", so a String there is already beside the point. For a plain
`ch_sendexpr()` with no callback, the value is read once into a local that
nothing goes on to use, and the dictionary is encoded as it stands.

That last case is the one that matters here. A message answering a request
the server made carries the server's id back and waits for nothing. Vim has
no reason to look at it, but the type check rejects it before it gets that
far, so a server naming its requests with a string cannot be answered at all.
python-lsp-server does exactly that: its `workspace/applyEdit` arrives with
`"id": "258cf855-32f3-4cb2-8f33-86215a3e4b25"`, and answering it gives E475.

The change would be to move the `callback_present` test above the type check
and require a Number only when Vim has to track the message: DAP, `eval`, or
a callback. Otherwise pass the dictionary through and leave the local id at
zero. About ten lines, plus a test that a String id is answered as it came.

If that sounds right I will prepare it, and this doc patch can either go or
say the opposite.

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