Add clear button with enhanced accessibility to Todo widget in chat (#259254)

* Initial plan

* Add clear button to Todo widget with trash icon

Co-authored-by: digitarald <[email protected]>

* Enhance clear button with visibility logic and finalize implementation

Co-authored-by: digitarald <[email protected]>

* Enhanced clear button accessibility and visual styling

Co-authored-by: digitarald <[email protected]>

* Wording nit

* Improve todo widget clear button: move to header with hover/focus visibility and accessibility

* Visually align line heights and padding; using the newly won space for longer todos

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: digitarald <[email protected]>
Co-authored-by: Harald Kirschner <[email protected]>