Update HintsEditor layout and collapse behavior

[Abhishek-Punhani]

Updated HintsEditor layout according to new figma specs and added collapse behavior

Summary

Changes:

• Added Collapsible header with new heading content "Hints (optional)"
• Each hint is now rendered as plain input in view mode and toggles to TipTapEditor in edit mode.
• The current "New hint" button is replaced with a dashed-border + Add hint button, centered and full-width.

References

fixes #5914

Reviewer guidance

1. Open a channel in the Studio channel editor.
2. Select or create an Exercise node.
3. In the edit panel, navigate to the Questions tab.
4. Open a question — the Hints section appears at the bottom of the question card, collapsed by default.

AI usage

Used Antigravity for final review and nitpicks.

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[Abhishek-Punhani]

@AlexVelezLl , Could you guide me how can I change the icon color in KButton ? in Add hint
Also the figma spec doesnt have assessmentToolBar?