test: add unit and integration tests for new features

50 unit tests covering new client methods, task serialization, error
paths, payload verification, and edge cases. 12 integration tests for
RecaptchaV2 and RecaptchaV3 against live demo pages (skipped without
API_KEY env var).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alperensert