[Testing]: Unit tests for gcode/setup.py key validation and env loading #59

Problem

setup.py handles first-run key setup, but its validation logic (200 vs 401
vs 403 vs network error) and .env loading are untested — exactly the paths
new users hit first.

Proposed approach

• Mock requests.get to cover: 200 → valid, 401 → invalid-key message,
  403 → forbidden message, ConnectionError/timeout → network message.
• Test load_env (reads ~/.gcode/.env when present, no-op otherwise),
  get_api_key precedence, and save_api_key round-trip.

Where to look

• gcode/setup.py:27-60 (get_api_key, load_env, validate_api_key),
  :80-89 (save_api_key)
• Pattern: tests/test_models.py mock-based tests.

Acceptance criteria

• New tests/test_setup.py covering the matrix above; all pass.

Difficulty

Low — good first issue.