E2609 : Add username search and participant addition in Calibration Tab

[ruju4a]

This PR implements a username search field within the Calibration Tab of the AssignmentEditor.
This allows instructors to search for specific users by their username and add them as calibration participants directly from the editor UI.

Changes:

1. Username Search Field: Added a new search input and "Add" button in the Calibration tab using Formik and React-Bootstrap InputGroup.
2. Participant Management: Implemented usernameSearch state and handleAddClick handler to manage user lookup and addition.
3. Table Enhancements: Updated the calibration participants table to robustly handle various participant name formats (e.g., username, fullName, nested user objects)