Messages API reports reasoning in order and the message phase

Model reasoning is now stored as ordered parts of a message instead of two flat strings. GET /v1/sessions/{id}/messages returns reasoning as ordered reasoning content parts, with provider replay state removed, and adds phase (commentary or final_answer) and phase_source (provider or derived).

Replay to providers is also more accurate: Anthropic thinking blocks keep their own signatures, including redacted_thinking; OpenAI reasoning items keep the ids the provider issued; and the Gemini driver now requests, parses and replays thoughts.

See Event reference, #3276 and #3279.