feat: support first-class /retry via API

[duongynhi000005-oss]

Summary

• add API::retry(conversation_id) as a first-class API path
• retry now reuses the last non-droppable user message (raw payload when present)
• wire both interactive /retry and conversation retry <id> to the same path
• add Conversation::last_user_message() with test coverage

Why

Closes #389

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