fix: add Anthropic computer-use beta header - #13242

I rechecked the current red claude-review job. The actual Phoenix checks are green, including Python required, TypeScript required, Playwright required, docs, schema, unit and integration jobs.

The remaining failure is in the Claude review workflow itself: it cannot get an OIDC token (Unable to get ACTIONS_ID_TOKEN_REQUEST_URL; likely missing id-token: write). I do not see a branch-side code action for that failure.

Rebased this PR onto current main and force-pushed the cleaned two-commit branch.

I rechecked the old red Python CI run more closely. The previous unit failures were from the May 27 merge ref in unit/server/agents/pydantic_ai/test_openinference_model_wrapper.py, not from the touched playground client helper tests. Rebasing should give this PR a fresh CI signal on the current base.

Local validation on Windows after the rebase:

python -m py_compile src\phoenix\server\api\helpers\playground_clients.py tests\unit\server\api\helpers\test_playground_clients.py
git diff --check upstream/main..HEAD

I attempted the targeted uv run ruff/pytest commands, but uv did not finish environment synchronization within the local timeout and left no test failure output. I stopped the leftover uv processes and did not change project files for the local environment issue.