Preserve user message structure in generic system squash

[biefan]

Summary

• squash the first system message into the first user message instead of rewriting whichever message happens to come second
• preserve multipart user messages when squashing system instructions into user content
• add regression coverage for assistant-before-user conversations and multipart user messages

Problem

GenericSystemSquashNormalizer currently assumes messages[1] is the user message and rebuilds it with Message.from_prompt(). That causes two issues:

• assistant messages that appear before the first user message get rewritten as user
• multipart user messages lose non-text pieces when the normalizer collapses them into a single text prompt

Validation

• .venv/bin/pytest tests/unit/message_normalizer -q
• .venv/bin/ruff check pyrit/message_normalizer/generic_system_squash.py tests/unit/message_normalizer/test_generic_system_squash_normalizer.py