DeriveUpstreamEndpoint hard-codes /v1/responses for PlatformOpenAI,
but APIKey accounts probed to not support Responses API are forwarded
directly to /v1/chat/completions via forwardAsRawChatCompletions.
Add resolveRawCCUpstreamEndpoint which returns /v1/chat/completions
when the account's extra.openai_responses_supported is explicitly false.