[ty] Initial support for workspace diagnostics (#18939)
## Summary
This PR adds initial support for workspace diagnostics in the ty server.
Reference spec:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_diagnostic
This is currently implemented via the **pull diagnostics method** which
was added in the current version (3.17) and the server advertises it via
the `diagnosticProvider.workspaceDiagnostics` server capability.
**Note:** This might be a bit confusing but a workspace diagnostics is
not for a single workspace but for all the workspaces that the server
handles. These are the ones that the server received during
initialization. Currently, the ty server doesn't support multiple
workspaces so this capability is also limited to provide diagnostics
only for a single workspace (the first one if the client provided
multiple).
A new `ty.diagnosticMode` server setting is added which can be either
`workspace` (for workspace diagnostics) or `openFilesOnly` (for checking
only open files) (default). This is same as
`python.analysis.diagnosticMode` that Pyright / Pylance utilizes. In the
future, we could use the value under `python.*` namespace as fallback to
improve the experience on user side to avoid setting the value multiple
times.
Part of: astral-sh/ty#81
## Test Plan
This capability was introduced in the current LSP version (~3 years) and
the way it's implemented by various clients are a bit different. I've
provided notes on what I've noticed and what would need to be done on
our side to further improve the experience.
### VS Code
VS Code sends the `workspace/diagnostic` requests every ~2 second:
```
[Trace - 12:12:32 PM] Sending request 'workspace/diagnostic - (403)'.
[Trace - 12:12:32 PM] Received response 'workspace/diagnostic - (403)' in 2ms.
[Trace - 12:12:34 PM] Sending request 'workspace/diagnostic - (404)'.
[Trace - 12:12:34 PM] Received response 'workspace/diagnostic - (404)' in 2ms.
[Trace - 12:12:36 PM] Sending request 'workspace/diagnostic - (405)'.
[Trace - 12:12:36 PM] Received response 'workspace/diagnostic - (405)' in 2ms.
[Trace - 12:12:38 PM] Sending request 'workspace/diagnostic - (406)'.
[Trace - 12:12:38 PM] Received response 'workspace/diagnostic - (406)' in 3ms.
[Trace - 12:12:40 PM] Sending request 'workspace/diagnostic - (407)'.
[Trace - 12:12:40 PM] Received response 'workspace/diagnostic - (407)' in 2ms.
...
```
I couldn't really find any resource that explains this behavior. But,
this does mean that we'd need to implement the caching layer via the
previous result ids sooner. This will allow the server to avoid sending
all the diagnostics on every request and instead just send a response
stating that the diagnostics hasn't changed yet. This could possibly be
achieved by using the salsa ID.
If we switch from workspace diagnostics to open-files diagnostics, the
server would send the diagnostics only via the `textDocument/diagnostic`
endpoint. Here, when a document containing the diagnostic is closed, the
server would send a publish diagnostics notification with an empty list
of diagnostics to clear the diagnostics from that document. The issue is
the VS Code doesn't seem to be clearing the diagnostics in this case
even though it receives the notification. (I'm going to open an issue on
VS Code side for this today.)
https://github.com/user-attachments/assets/b0c0833d-386c-49f5-8a15-0ac9133e15ed
### Zed
Zed's implementation works by refreshing the workspace diagnostics
whenever the content of the documents are changed. This seems like a
very reasonable behavior and I was a bit surprised that VS Code didn't
use this heuristic.
https://github.com/user-attachments/assets/71c7b546-7970-434a-9ba0-4fa620647f6c
### Neovim
Neovim only recently added support for workspace diagnostics
(https://github.com/neovim/neovim/pull/34262, merged ~3 weeks ago) so
it's only available on nightly versions.
The initial support is limited and requires fetching the workspace
diagnostics manually as demonstrated in the video. It doesn't support
refreshing the workspace diagnostics either, so that would need to be
done manually as well. I'm assuming that these are just a temporary
limitation and will be implemented before the stable release.
https://github.com/user-attachments/assets/25b4a0e5-9833-4877-88ad-279904fffaf9
