Update known models on Cerebras and Heroku

[Kludex]

Note

Updates KnownModelName with new Heroku and Cerebras entries and removes deprecated Cerebras models; test cassettes updated accordingly.

• Models (pydantic_ai_slim/pydantic_ai/models/__init__.py):
  • Add: cerebras:zai-glm-4.6, heroku:amazon-rerank-1-0, heroku:claude-4-5-haiku, heroku:claude-4-5-sonnet, heroku:cohere-rerank-3-5 to KnownModelName.
  • Remove: cerebras:llama-4-scout-17b-16e-instruct, cerebras:llama-4-maverick-17b-128e-instruct, cerebras:qwen-3-coder-480b.
• Tests (tests/models/cassettes/test_model_names/test_known_model_names.yaml):
  • Update Heroku available models to include the new entries above; expand regions (e.g., gpt-oss-120b includes eu).
  • Update Cerebras models list to drop removed models and include zai-glm-4.6 (reordered list).

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