fix(mistral): inject dummy user when last turn is assistant

[leilei3167]

Mistral's Conversations API returns 400 / code 3230 when the last turn is an assistant message. That's a valid LiveKit context after with_filler() or generate_reply() with no user_input.

The other formatters already inject a dummy user turn for this. Mistral's path never took inject_dummy_user_message, so the request went up as-is.

Fixes #7030

Test plan

• uv run pytest tests/test_chat_ctx.py --unit (61 passed, 1 skipped)
• ruff check / ruff format on the touched files

[leilei3167]

@livekit/agent-devs small one for #7030 — Mistral was dropping inject_dummy_user_message, so a trailing assistant turn (filler / generate_reply() with no user_input) 400'd. Same dummy . user turn the Google formatter already uses.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.