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/proto/proto/lsp.proto

@@ -195,6 +195,8 @@ message LspExtGoToParentModuleResponse {
 message GetCompletionsResponse {
     repeated Completion completions = 1;
     repeated VectorClockEntry version = 2;
+    // `!is_complete`, inverted for a default of `is_complete = true`
+    bool can_reuse = 3;
 }
 
 message ApplyCompletionAdditionalEdits {