Add unit tests for zhtw tool parameter edge cases

## Summary
`handle_tools_call` in `mcp/tools.rs` lacks unit tests for parameter combination edge cases.

## Coverage needed
- Empty text input
- `max_errors=0` gate behavior
- Invalid `content_type`
- Unknown profile name
- `fix_mode` combined with `political_stance`
- `explain` combined with `compact` output
- Gate: ≥12 unit tests across parameter-combination edge cases.