feat: add LiteLLM provider adapter for judge metrics (#103)

[decko]

Summary

• Adds litellm to LLMProvider Literal type
• Adds litellm optional dependency in pyproject.toml
• New dispatch branch in create_ragas_llm() for LiteLLM provider
• patch_litellm_for_token_tracking() captures token usage from LiteLLM responses
• 255 lines of mock-based tests (no real API calls)

Test plan

• uv run pytest tests/test_metrics_ragas.py -v — 168 passed
• LiteLLM dispatch creates correct client
• Token tracking works with LiteLLM response format
• Existing anthropic/vertex-anthropic paths unchanged

Refs #103

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