Update LLM Model List

Overview

The canonical model list lives in sdks/python/agenta/sdk/utils/assets.py → supported_llm_models. It drives the model dropdown in the playground, cost metadata, and the model_to_provider_mapping.

The authoritative external source is litellm.model_cost (2 600+ entries), which mirrors https://models.litellm.ai/.

A pytest guard lives at: sdks/python/oss/tests/pytest/unit/test_supported_llm_models.py

Key rules