Display loader and disable input while explain code is being is being generated
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### Summary
In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/143217 we updated the Explain code feature to use the Duo Chat component.
This issue suggests displaying a loader and disabling user input while the answer is being generated.
### Proposal
| before | after |
| ------ | ------ |
|  |  |
issue