feat: Nexent规格表-enforce conversation turn and history limits

[Junqilee]

Summary

• Limit each conversation to 100 user turns.
• Limit each user's active conversation history to 1,000 conversations.
• Enforce limits in both conversation creation and Agent execution paths.
• Return standard error code 120104 with HTTP 429.
• Add structured error details and Chinese/English frontend messages for both /chat and /newchat.

Validation

• Backend database, service, conversation API, and Agent API tests passed.
• Frontend build passed.
• Manually verified Chinese and English limit messages in the isolated environment.
• Formal limits restored to 100/1,000 before commit.

前端测试时，临时设置limit均为1（仅支持1个历史会话，每个会话仅支持1轮）。效果见以下截图：

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/database/conversation_db.py	90.00%	3 Missing ⚠️
...ackend/services/conversation_management_service.py	90.90%	1 Missing ⚠️

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