fix(auto-drive): salvage user-turn json (#353)

[zemaj]

Summary

• reuse the existing extract_first_json_object helper for user-turn replies so mixed LLM chatter still yields JSON
• ensure parse_user_turn_reply returns the salvaged object while keeping all current validation/trim semantics intact
• cover both the strict object and salvage flows with unit tests to catch regressions

Testing

• cargo test -p code-auto-drive-core coordinator_user_schema
• ./build-fast.sh

Closes #353.