/v1/models is shared by the OpenAI and Anthropic surfaces and was
dispatched to the Claude format only for claude-cli User-Agents, so
generic Anthropic API clients received the OpenAI format. Distinguish
Anthropic requests by the Anthropic-Version header (sent by every
Anthropic client, never by OpenAI/Codex clients) in addition to the
existing claude-cli check, in both unifiedModelsHandler and
handleHomeModels. Additive and non-breaking for existing clients.