fix(computer-use): reject nonpositive image dimensions and limits

[00200200]

The image-size search assumes a valid one-pixel lower bound. Nonpositive limits violate that assumption: a one-pixel image with max_tokens=0 never terminates, and larger inputs can return dimensions that exceed the requested cap. Reject nonpositive dimensions and sizing limits before the search. The hang regression runs in a child process with a timeout; ordinary valid sizing cases remain covered.

Validation: PYTHONPATH=computer-use-best-practices/ python -m pytest computer-use-best-practices/tests/test_image.py -q (18 passed in 0.54s). The regression tests fail on the unchanged main branch. Tests use local fixtures; no paid API calls.

Additional checks: ruff check computer-use-best-practices/tests/test_image.py computer-use-best-practices/computer_use/image.py, ruff format --check computer-use-best-practices/tests/test_image.py computer-use-best-practices/computer_use/image.py.

Prepared with Codex assistance; the final diff and regression behavior were checked locally.