Anthropic retired Claude Sonnet 4 (claude-sonnet-4-0 /
claude-sonnet-4-20250514) on 2026-06-15. The live-API model test in
tests/ci/models/test_llm_anthropic.py pinned that alias, so every CI run
now gets a 404 not_found_error and the agent never completes the task,
failing the models/test_llm_anthropic job on all branches.
Bump the test to the current GA Sonnet (claude-sonnet-4-6).