Avoid modifying the LSP message before resolving it (#26347)

Closes https://github.com/zed-industries/zed/issues/21277

To the left is current Zed, right is the improved version.
3rd message, from Zed, to resolve the item, does not have `textEdit` on
the right side, and has one on the left.
Seems to not influence the end result though, but at least Zed behaves
more appropriate now.

<img width="1727" alt="image"
src="https://github.com/user-attachments/assets/ca1236fd-9ce2-41ba-88fe-1f3178cdcbde"
/>


Instead of modifying the original LSP completion item, store completion
list defaults and apply them when the item is requested (except `data`
defaults, needed for resolve).

Now, the only place that can modify the completion items is this method,
and Python impl seems to be the one doing it:


ca9c3af56f/crates/languages/src/python.rs (L182-L204)

Seems ok to leave untouched for now.

Release Notes:

- Fixed LSP completion items modified before resolve request

crates/proto/proto/zed.proto

@@ -1000,6 +1000,7 @@ message Completion {
     bytes lsp_completion = 5;
     bool resolved = 6;
     Source source = 7;
+    optional bytes lsp_defaults = 8;
 
     enum Source {
         Custom = 0;