Add validation for Mastodon handles to prevent invalid URLs

[marcoacierno]

Summary

This PR adds validation for Mastodon handles to fix the issue where invalid handles like just "username" result in broken URLs (https://undefined/@username).

Changes

• Added Mastodon handle validation in participant, submission, and grant mutations
• Enforces proper formats: [email protected], @[email protected], or https://instance.social/@username
• Added test case to verify validation works correctly

Fixes #3363

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Project	Deployment	Preview	Updated (UTC)
pycon	Ready	Preview	Oct 3, 2025 11:39pm

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Codecov Report

:x: Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 94.46%. Comparing base (6686fae) to head (4b6746d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4483      +/-   ##
==========================================
- Coverage   94.53%   94.46%   -0.08%     
==========================================
  Files         352      352              
  Lines       10131    10147      +16     
  Branches      733      738       +5     
==========================================
+ Hits         9577     9585       +8     
- Misses        461      466       +5     
- Partials       93       96       +3     

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