test: replace remaining contract placeholders with executable suites

[Patrik652]

Summary

• replace tests/contract/test_vision_service.py with executable contract tests using deterministic doubles
• replace tests/contract/test_claude_api_client.py with executable Anthropic client contract tests using mocked API responses
• remove the last placeholder-driven contract skips and keep behavior covered by runnable tests

Test Plan

• PYTHONPATH=. .venv/bin/ruff check src/services src/cli tests/contract/test_vision_service.py tests/contract/test_claude_api_client.py
• PYTHONPATH=. .venv/bin/mypy src/services src/cli
• PYTHONPATH=. .venv/bin/pytest -q

Result

• 317 passed, 0 skipped, 2 warnings