[8.15] Fix support for IME in Assistant prompt (#184874) (#188384)

# Backport

This will backport the following commits from `main` to `8.15`:
- [Fix support for IME in Assistant prompt
(#184874)](#184874)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Patryk
Kopyciński","email":"[email protected]"},"sourceCommit":{"committedDate":"2024-07-16T00:21:59Z","message":"Fix
support for IME in Assistant prompt (#184874)\n\n## Summary\r\n\r\nKudos
to @sakurai-youhei for providing a fix 🙇\r\n \r\nTested on Chrome,
Firefox,
Safari\r\n\r\n\r\nhttps://github.com/elastic/kibana/assets/5188868/f0ff388f-3943-4382-a873-88949b760629\r\n\r\n---------\r\n\r\nCo-authored-by:
Garrett Spong
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support for IME in Assistant prompt (#184874)\n\n## Summary\r\n\r\nKudos
to @sakurai-youhei for providing a fix 🙇\r\n \r\nTested on Chrome,
Firefox,
Safari\r\n\r\n\r\nhttps://github.com/elastic/kibana/assets/5188868/f0ff388f-3943-4382-a873-88949b760629\r\n\r\n---------\r\n\r\nCo-authored-by:
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support for IME in Assistant prompt (#184874)\n\n## Summary\r\n\r\nKudos
to @sakurai-youhei for providing a fix 🙇\r\n \r\nTested on Chrome,
Firefox,
Safari\r\n\r\n\r\nhttps://github.com/elastic/kibana/assets/5188868/f0ff388f-3943-4382-a873-88949b760629\r\n\r\n---------\r\n\r\nCo-authored-by:
Garrett Spong
<[email protected]>","sha":"87f0e71d193ec77912cc4b99f52cc7fdcf2dc4bc"}}]}]
BACKPORT-->

Co-authored-by: Patryk Kopyciński <[email protected]>

x-pack/packages/kbn-elastic-assistant/impl/assistant/prompt_textarea/index.tsx

@@ -30,13 +30,16 @@ export const PromptTextArea = forwardRef<HTMLTextAreaElement, Props>(
 
     const onKeyDown = useCallback(
       (event) => {
-        if (event.key === 'Enter' && !event.shiftKey && value.trim().length > 0) {
+        // keyCode 13 is needed in case of IME input
+        if (event.keyCode === 13 && !event.shiftKey) {
           event.preventDefault();
-          onPromptSubmit(event.target.value?.trim());
-          handlePromptChange('');
-        } else if (event.key === 'Enter' && !event.shiftKey && value.trim().length === 0) {
-          event.preventDefault();
-          event.stopPropagation();
+
+          if (value.trim().length) {
+            onPromptSubmit(event.target.value?.trim());
+            handlePromptChange('');
+          } else {
+            event.stopPropagation();
+          }
         }
       },
       [value, onPromptSubmit, handlePromptChange]