Preserve roles when converting messages to seed prompts

[biefan]

Summary

• preserve API-compatible message roles when converting Message objects to SeedPrompt records
• keep the existing simulated_assistant -> assistant normalization by relying on message.api_role
• add regression coverage for system, developer, tool, and simulated_assistant

Problem

SeedPrompt.from_messages() currently collapses every non-assistant message role to user. That loses supported roles such as system, developer, and tool when messages are converted into seed prompts.

Validation

• .venv/bin/pytest tests/unit/models/test_seed.py -q
• .venv/bin/ruff check pyrit/models/seeds/seed_prompt.py tests/unit/models/test_seed.py

[romanlutz]

This looks correct. I'm wondering why we had that here...