Fix W&B tracker initialization handling

[bghira]

Summary

• make tracker initialization idempotent so repeated entrypoints do not register duplicate trackers
• surface tracker initialization failures instead of continuing with only SimpleTuner metrics
• use a fresh persisted W&B run id for new runs and reuse it on checkpoint resume

Tests

• .venv/bin/python -m unittest tests.test_trainer tests.test_report_to tests.test_local_metrics -v -f
• .venv/bin/python -m unittest -v -f