Fix seeding in training workflow

[gitttt-1234]

This PR fixes seeding in the training pipeline. Previously, the seed parameter in the config was defined but never applied. We now explicitly set the seed when provided. In multi-GPU setups, if validation labels are missing, we default the seed to 42 during train/val splitting to ensure all workers use the same split.

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.61%. Comparing base (ff91433) to head (3072ec9).
⚠️ Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_nn/training/model_trainer.py	71.42%	8 Missing ⚠️

❌ Your patch status has failed because the patch coverage (72.41%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
- Coverage   95.28%   93.61%   -1.68%     
==========================================
  Files          49       49              
  Lines        6765     6921     +156     
==========================================
+ Hits         6446     6479      +33     
- Misses        319      442     +123     

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