chore: improve rich maintenance path

[lphuc2250gma]

Summary:

• Add edge-case unit tests to tests/test_cells.py covering cell_len/set_cell_size/chop_cells behavior for empty strings, strings containing zero-width joiner sequences, mixed wide/narrow CJK characters, and set_cell_size with total exactly equal to current cell length (boundary). Tighten type annotations in rich/cells.py if any function parameters or returns are missing precise types (e.g., Iterable vs List).
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.