Fix LSP violation when dismissing server notifications (#23531)
Part of https://github.com/zed-industries/zed/issues/22606
Closes https://github.com/zed-industries/zed/issues/23509
When a user sees an odd notification from the language server like
<img width="508" alt="image"
src="https://github.com/user-attachments/assets/6f5ef1aa-0f09-4705-a02a-aaf81dd8620c"
/>
they usually dismiss that.
Zed uses channels to wait and handle user interactions with such
notifications, and, due to `?`, sends back
```json
{"jsonrpc":"2.0","id":1,"error":{"message":"receiving from an empty and closed channel"}}
```
which is not spec-compliant:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#window_showMessageRequest
> Response:
>
> * result: the selected
[MessageActionItem](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#messageActionItem)
| null if none got selected.
> * error: code and message set in case an exception happens during
showing a message.
Unfortunately, vtsls (and, potentially, others) crash if receive such
non-compliant requests, and do not get back.
After the fix, the message is correct:
```json
{"jsonrpc":"2.0","id":1,"result":null}
```
Release Notes:
- Fixed vtsls crashing on notification dismiss
Co-authored-by: Piotr Osiewicz <piotr@zed.dev>
95c045ad93