fix(anthropic): build message params as plain dicts so anthropic 2.x works - #1955

anthropic 2.0.0 (2026-10-21) changed anthropic.types.MessageParam into a
Union[UserMessageParam, AssistantMessageParam, SystemMessageParam], so calling it now raises

TypeError: Cannot instantiate typing.Union

evalscope doesn't pin anthropic, so anyone with a current install hits this on every
--eval-type anthropic_api run, before any request goes out. The calls are all in
evalscope/models/utils/anthropic.py (anthropic_message_param and _combine_messages).

This swaps the 7 MessageParam(...) calls for dict literals. The TypedDict "constructor" only
ever returned a dict, so the output is the same on older SDKs.

I checked the sampling params too: on 2.x messages.create no longer has temperature, top_p
or top_k, but validate_request_params already moves anything not in the signature into
extra_body, so those still get sent and don't raise. I left that alone.

Testing (Python 3.13, Windows):

• tests/models/test_anthropic_*.py + tests/agent/external/test_anthropic_bridge_prompt_cache.py
  • main + anthropic 2.3.0: 17 failed, 27 passed
  • this branch + anthropic 2.3.0 / 1.17.0 / 0.75.0: 44 passed each
• AnthropicCompatibleAPI.generate() with temperature=0.0 against a dead local port:
  main fails with the TypeError, this branch gets to the HTTP request (connection error, as expected)
• ruff check and ruff format --check clean