Fix parsing of old format contact field in wallet

[callebtc]

This pull request fixes an issue where the contact field in the wallet was not being parsed correctly if it was in the old format. The issue was identified in commit "wallet parse contact field if its old format". The problem has been resolved by adding a root validator to preprocess the deprecated contact field and convert it to the new format. This ensures that the contact field is parsed correctly in all cases.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 63.96%. Comparing base (f32099b) to head (a9f8717).
Report is 2 commits behind head on main.

Files	Patch %	Lines
cashu/core/models.py	71.42%	2 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (f32099b) and HEAD (a9f8717). Click for more details.

HEAD has 7 uploads less than BASE

Flag	BASE (f32099b)	HEAD (a9f8717)
	11	4

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #589      +/-   ##
==========================================
- Coverage   69.01%   63.96%   -5.05%     
==========================================
  Files          92       92              
  Lines        8658     8669      +11     
==========================================
- Hits         5975     5545     -430     
- Misses       2683     3124     +441     

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