[OSPP] Implement Full Context Management for the Dubbo Admin Agent

为Dubbo Admin Agent 实现完整的上下文管理功能：
1. Prompt Template的占位和渲染。System Prompt, AI Message, User Message 的组合。
2. 长对话的上下文压缩。
3. 跨对话的记忆机制。

更多细节请参考：https://summer.ospp.ac.cn/org/prodetail/26cf40055?list=org&navpage=org

> English version

Implement comprehensive context management for the Dubbo Admin Agent:
1. Prompt template placeholders and rendering, including the composition of system prompts, AI messages, and user messages.
2. Context compression for long conversations.
3. A cross-conversation memory mechanism.
For more details, please refer to: https://summer.ospp.ac.cn/org/prodetail/26cf40055?list=org&navpage=org

No source files or tests are named in the issue. Start by reading the linked OSPP proposal and locating the Dubbo Admin Agent entry point; map prompt rendering, conversation history, compression, and memory boundaries before defining tests for each capability. Done means all three requested behaviors are implemented and covered by tests.

Written by the indexing model from the issue text.