Bump chat UI to v2 with SDK v6 protocol

[bendrucker]

Bumps CHAT_UI_VERSION to 2.0.0 and hardcodes sdk_version=6 for the Vercel AI adapter. V2 of the UI requires SDK v6 for tool approval and denied tool states. Depends on pydantic/ai-chat-ui#16.

test_chat_app_index_endpoint and test_chat_app_index_caching will fail until @pydantic/ai-chat-ui@2.0.0 is published to the CDN.

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• PR title is fit for the release changelog.

Pre-Merge Checklist

• New tests for any fix or new behavior, maintaining 100% coverage.
• Updated documentation for new features and behaviors, including docstrings for API docs.