Add Anthropic prefill messages

[caydyan]

/claim #28

Summary

• Move devcontainer generation message construction into a focused helper.
• Preserve the existing system + user messages for non-Anthropic models.
• Add an assistant prefill message when MODEL is a Claude/Anthropic model so Anthropic-style prefill prompting can be tested without changing the default Azure/OpenAI path.
• Make the prefill configurable with optional ANTHROPIC_PREFILL_PROMPT, defaulting to {.
• Document the optional setting and add unit coverage for Anthropic and non-Anthropic message payloads.

Tests

python3 -m unittest tests/test_llm_message_helpers.py
python3 -m unittest discover -s tests
python3 -m compileall helpers main.py tests/test_llm_message_helpers.py
git diff --check

Repository note

Algora links this bounty to daytonaio/devcontainer-generator#28, but that GitHub repo path currently returns 404/disabled issues. This PR targets nibzard/devcontainer-generator, matching the existing claim PR pattern in this repo.