The test was asserting messagesJson[0].content[0].text but rawMessageToCAPI
converts single text content parts to a plain string, so content is a string
not an array. Updated assertion to check messagesJson[0].content directly.
Also added telemetry emission (sendEngineMessagesTelemetry) to
processResponseFromChatEndpoint so engine.messages events are sent for
Responses API completions.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>