[Fix] Update import paths in engine-explorer and _app.tsx for better modularity (#3547)
### TL;DR
Corrected stylesheet imports for `swagger-ui.css` in the Engine Explorer component and added the Toaster component import in the _app.tsx.
### What changed?
- Added missing `swagger-ui.css` import in `engine-explorer.tsx`
- Removed duplicate `swagger-ui.css` import from `_app.tsx`
- Ensured Toaster component import in `_app.tsx`
### How to test?
- Verify UI for the Engine Explorer relies on the correct styles
- Ensure the Toaster component renders correctly without issues
### Why make this change?
To ensure the correct CSS styles are applied and to improve the maintainability of the import statements.
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## PR-Codex overview
This PR adds `swagger-ui.css` to `engine-explorer.tsx` and removes it from `_app.tsx`.
### Detailed summary
- Added `swagger-ui.css` import to `engine-explorer.tsx`
- Removed `swagger-ui.css` import from `_app.tsx`
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