ruff server: Support publish diagnostics as a fallback when pull diagnostics aren't supported (#11092)

## Summary

Fixes #11059 

Several major editors don't support [pull
diagnostics](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_pullDiagnostics),
a method of sending diagnostics to the client that was introduced in
version `0.3.17` of the specification. Until now, `ruff server` has only
used pull diagnostics, which resulted in diagnostics not being available
on Neovim and Helix, which don't support pull diagnostics yet (though
Neovim `10.0` will have support for this).

`ruff server` will now utilize the older method of sending diagnostics,
known as 'publish diagnostics', when pull diagnostics aren't supported
by the client. This involves re-linting a document every time it is
opened or modified, and then sending the diagnostics generated from that
lint to the client via the `textDocument/publishDiagnostics`
notification.

## Test Plan

The easiest way to test that this PR works is to check if diagnostics
show up on Neovim `<=0.9`.

crates/ruff_server/src/session/capabilities.rs

@@ -7,6 +7,7 @@ pub(crate) struct ResolvedClientCapabilities {
     pub(crate) apply_edit: bool,
     pub(crate) document_changes: bool,
     pub(crate) workspace_refresh: bool,
+    pub(crate) pull_diagnostics: bool,
 }
 
 impl ResolvedClientCapabilities {
@@ -48,12 +49,19 @@ impl ResolvedClientCapabilities {
             .unwrap_or_default();
         */
 
+        let pull_diagnostics = client_capabilities
+            .text_document
+            .as_ref()
+            .and_then(|text_document| text_document.diagnostic.as_ref())
+            .is_some();
+
         Self {
             code_action_deferred_edit_resolution: code_action_data_support
                 && code_action_edit_resolution,
             apply_edit,
             document_changes,
             workspace_refresh,
+            pull_diagnostics,
         }
     }
 }