Normalize SeedPrompt file extension detection

[biefan]

Summary

• normalize file extensions before inferring SeedPrompt.data_type from existing files
• add regression coverage for uppercase image and audio extensions

Problem

SeedPrompt infers file-backed prompt types by reading the file extension, but the comparison is currently case-sensitive.

That makes valid files such as example.PNG or sample.WAV raise ValueError instead of being inferred as image_path or audio_path, even though the extension is otherwise supported.

Testing

• .venv/bin/pytest tests/unit/models/test_seed.py -q