Avoid re-querying language server completions when possible (#31872)

Also adds reuse of the markdown documentation cache even when
completions are re-queried, so that markdown documentation doesn't
flicker when `is_incomplete: true` (completions provided by rust
analyzer always set this)

Release Notes:

- Added support for filtering language server completions instead of
re-querying.

crates/remote_server/src/remote_editing_tests.rs

@@ -513,8 +513,8 @@ async fn test_remote_lsp(cx: &mut TestAppContext, server_cx: &mut TestAppContext
 
     assert_eq!(
         result
-            .unwrap()
             .into_iter()
+            .flat_map(|response| response.completions)
             .map(|c| c.label.text)
             .collect::<Vec<_>>(),
         vec!["boop".to_string()]