style(tui): reorder test sections to mirror production function order

- status.rs: swap set_status and tick sections (set_status precedes
  tick in production)
- theme.rs: move thinking_is_italic to composite helpers section
  (thinking() is a composite helper, not a text style)
- chat.rs: reorganize all test sections to follow production function
  order — append_stream_token → commit_streaming → update_layout →
  handle_event → render → scroll helpers → build_text → push_* helpers
  → advance_streaming_cache. Rename tests to match their section header
  function name. Move helper constructors (key_event, mouse_scroll) to
  top of module.

Author: hakula139