test: cover transcript parser edge cases

[haosenwang1018]

Summary

• add coverage for find_turn_context() matching when the lookup string is longer than the stored turn prefix
• add coverage for parse_transcript() safely ignoring assistant entries whose message.content is not a list
• keep the patch scoped to transcript parser helper branches under issue test: improve test coverage across the project #114

Testing

• uv run python -m pytest tests/test_transcript.py -q
• uv run ruff check tests/test_transcript.py
• uv run ruff format --check tests/test_transcript.py

Refs #114