feat(eval): add optional API Route reader

[DennyHo0917]

Summary

Add an optional API Route reader to the evaluation CLI, following the existing Cheaper Inference (#163) and Atlas Cloud (#159) reader integrations.

• Select explicitly with --api-route; the base URL is https://global.api-route.com/v1.
• Resolve only --api-key or API_ROUTE_API_KEY, preserving exact account model IDs. Other providers' keys are never used.
• Force the existing OpenAI-compatible client even for model IDs containing gemini.
• Reject other provider flags, LiteLLM and endpoint overrides when this reader is selected.
• Disable automatic request retries for this optional reader, as Atlas Cloud already does, to avoid duplicating billable generations.
• Add setup documentation and offline regression tests. Existing reader defaults and dependencies remain unchanged.

Validation

• pytest tests/test_api_route_client.py tests/test_atlascloud_client.py tests/test_cheaperinference_client.py -q: 29 passed.
• Actual run_bench.py --help succeeds and exposes --api-route.
• Actual CLI rejects all six competing provider flags and --api-base with exit code 2 before loading a dataset or generating an answer.
• Ruff lint and format checks passed for the three affected Python files; git diff --check passed.
• Windows validation used an isolated uv run --no-project --with ... environment because the project restricts its UV environments to Linux/macOS. No project dependency/lockfile change was needed.
• HTTP/SDK calls are mocked in the tests. No live inference, vision-model compatibility or end-to-end retrieval benchmark was tested; the docs require users to verify image support for screenshot readers.

Disclosure

I operate API Route. This contribution was prepared with AI assistance and checked locally. It adds no model context/pricing/capability metadata or default model recommendation.

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