test(conversation): add tests for save validation, custom vocabulary size, and empty file loading

[fabihy]

Five additional tests for conversation.py:

• save_conversations raises before writing when turns are invalid (file is not created)
• validate_example with custom vocabulary_size rejects out-of-range target IDs
• A user Turn can carry both text and audio simultaneously
• load_conversations with an empty file returns an empty list
• A single user turn is a valid conversation prefix

All 33 tests pass locally (pytest tests/test_conversation.py -q).