test(alerts): add deeply nested collection null byte sanitization specs

[gcoinstash-cmd]

Summary

• Extends test parameters in tests/test_alert_utils.py to verify that sanitize_alert recursively strips null bytes (\x00) across deeply nested dictionaries containing embedded lists of objects.
• Adds regression protection for empty dictionary inputs.

Testing

• Verifies recursive sanitization and structural equality.

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[shahargl]

ai slop