CupertinoSliverNavigationBar.search fidelity updates (#163089)

## Native iOS overlaid on Flutter view

| Inactive search state | Active search state | 
| --- | --- |
| <img alt="Screenshot 2025-02-11 at 3 31 40 PM"
src="https://github.com/user-attachments/assets/eefa539e-8e78-4047-8876-c16db26f59df"
/> | <img alt="Screenshot 2025-02-11 at 3 34 01 PM"
src="https://github.com/user-attachments/assets/2b4b9a20-5a99-493d-b343-ac38b039efa7"
/> |

Animation curve and duration values gotten from xcode.

Addresses some of the issues in
https://github.com/flutter/flutter/issues/163020