fix: preserve non-ASCII characters in JSON output

[jijiamoer]

Summary

• emit non-ASCII characters directly in CLI JSON output instead of escaping them as \uXXXX
• write JSON output files explicitly as UTF-8
• add regression coverage for both file and stdout output

Test plan

• uv run ruff check --fix .
• uv run ruff format .
• uv run tach check
• uv run ty check
• uv run pytest -q (843 passed)