chore: update deprecated validator to field_validator

[Oisin-M]

Description

Close #24

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.11%. Comparing base (47f963d) to head (3a62cb6).
⚠️ Report is 39 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (47f963d) and HEAD (3a62cb6). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (47f963d)	HEAD (3a62cb6)
	4	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #25       +/-   ##
===========================================
- Coverage   97.33%   63.11%   -34.23%     
===========================================
  Files           8        8               
  Lines         525      572       +47     
===========================================
- Hits          511      361      -150     
- Misses         14      211      +197     

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