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/extension_host/src/extension_store_test.rs

@@ -759,8 +759,8 @@ async fn test_extension_store_with_test_extension(cx: &mut TestAppContext) {
         })
         .await
         .unwrap()
-        .unwrap()
         .into_iter()
+        .flat_map(|response| response.completions)
         .map(|c| c.label.text)
         .collect::<Vec<_>>();
     assert_eq!(