[OPIK-5665] [FE] fix: allow collapsing Ollie sidebar in error state

[awkoy]

Details

When the compute endpoint returns 500 and the error state is shown, the Ollie sidebar was stuck open with no way to close it. This adds toggle functionality so the error state can be collapsed/expanded, matching the normal sidebar behavior.

• Added onToggle to AssistantErrorState — expanded error shows an X close button, collapsed error strip expands on click
• Made isOpen stateful in AssistantSidebarLoader so it can toggle, persisting to localStorage and updating layout width

Issues

https://comet-ml.atlassian.net/browse/OPIK-5665

Change checklist

• I have reviewed my own code
• I have added relevant tests or verified manually

Testing

• Force a compute 500 (hardcode DEV_RESULT to error state) and verify expanded error shows X button
• Click X — sidebar collapses to thin strip with error icon
• Click collapsed strip — sidebar expands back to full error view
• Retry button still works from expanded state
• Collapse state persists across page reloads
• Normal (non-error) sidebar loading/ready states unaffected

Documentation

No documentation changes needed.

[github-actions]

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