ruff server: In 'publish diagnostics' mode, document diagnostics are cleared properly when a file is closed (#11137)

## Summary

Fixes #11114. 

As part of the `onClose` handler, we publish an empty array of
diagnostics for the document being closed, similar to
[`ruff-lsp`](187d7790be/ruff_lsp/server.py (L459-L464)).
This prevent phantom diagnostics from lingering after a document is
closed. We'll only do this if the client doesn't support pull
diagnostics, because otherwise clearing diagnostics is their
responsibility.

## Test Plan

Diagnostics should no longer appear for a document in the Problems tab
after the document is closed.

crates/ruff_server/src/server/api/diagnostics.rs

@@ -32,3 +32,20 @@ pub(super) fn publish_diagnostics_for_document(
 
     Ok(())
 }
+
+pub(super) fn clear_diagnostics_for_document(
+    snapshot: &DocumentSnapshot,
+    notifier: &Notifier,
+) -> crate::server::Result<()> {
+    notifier
+        .notify::<lsp_types::notification::PublishDiagnostics>(
+            lsp_types::PublishDiagnosticsParams {
+                uri: snapshot.url().clone(),
+                diagnostics: vec![],
+                version: Some(snapshot.document().version()),
+            },
+        )
+        .with_failure_code(lsp_server::ErrorCode::InternalError)?;
+
+    Ok(())
+}