Move litellm to an optional extra

[aniketwattamwar]

Problem

runtime/python/pyproject.toml has litellm>=1.0.0,<1.50.0 as a hard dependency, pulled in solely for the llm_judge grader. Every user installs a heavy transitive tree whether or not they ever use a judge, and our version pin becomes theirs.

It is also why test_llm_judge_fails_without_api_key and test_llm_judge_uses_configured_model fail on a clean checkout without it.

Proposal

[project.optional-dependencies]
judge = ["litellm>=1.0.0,<1.50.0"]

graders.py already handles the import being absent and returns a clear "install with pip install litellm" message — so the fallback path exists and is tested.

Acceptance criteria

• litellm moved to a judge extra
• ecp run without it gives an actionable error only when a manifest uses llm_judge
• Tests skip cleanly rather than fail when absent
• Install docs updated across README, docs, and both package READMEs