fix(chatflow): support assistant role and fix message ordering for additional_messages

[sharmt1411]

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fix

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fix(chatflow): additional_messages支持 assistant 角色并修复消息顺序问题

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en: This PR fixes several issues in chatflow related to additional_messages handling:

Support Multiple Roles: Previously, roles in additional_messages were hardcoded to user. This PR enables proper mapping to support roles like assistant.
Fix Message Duplication: Adjusted the creation sequence so that history messages (from additional_messages) are created before the current run and user message. This ensures their CreatedAt timestamps are earlier, allowing prefetchChatHistory to correctly skip existing messages and preventing duplication.
Guaranteed Sequential Order: Implemented manual timestamp management for history messages and switched to BatchCreate to ensure strict chronological order when stored in the database.

zh: 此 PR 修复了 chatflow 处理 additional_messages 时的几个关键问题：

支持多种角色：修复了之前 additional_messages 中角色被硬编码为 user 的问题，现在能正确识别并支持 assistant 等角色。
修复消息重复：调整了创建顺序，确保 additional_messages 中的历史消息在当前最新轮次user 的 Run Record 和消息之前创建。这保证了历史消息的 CreatedAt 时间戳更早，从而让 prefetchChatHistory 的跳过逻辑能正常工作，避免了取回历史消息时出现的重复问题。
保证消息顺序：为历史消息引入了手动的时间戳增量逻辑，并从并发创建改为 BatchCreate（批量创建），确保消息在数据库中严格按序排列，防止获取最新消息时顺序错乱。
代码变更要点回顾：
角色映射：在 toConversationMessage 中将 Role: schema.User 修改为 Role: schema.RoleType(msg.Role)。
创建时序：将 makeChatFlowHistoryMessages 和 BatchCreate 的调用移到了 OpenAPIChatFlowRun 的最开始部分。
时间戳管理：在 makeChatFlowHistoryMessages 中通过 baseTimestamp + int64(i*100) 为每条历史消息分配了递增的时间戳。
性能优化：移除了 taskgroup 的并发创建，改用一次 BatchCreate 提交，既保证了顺序又提高了效率。

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