fix: include prepend context in gateway recall response

[lazyayuan]

Description | 描述

Fix a Gateway/Hermes /recall compatibility issue where L1 recall content in prependContext could be dropped.

The core recall logic already returns L1 memories in prependContext, but the Gateway /recall response previously only exposed appendSystemContext through the legacy context field. As a result, custom Agent integrations or the Hermes provider calling Gateway /recall directly could not receive the L1 memories returned in prependContext.

This PR:

• Adds prependContext and appendSystemContext to the Gateway /recall response.
• Preserves backward compatibility by building the legacy context field from appendSystemContext + prependContext.
• Updates Hermes prefetch() to prefer the split fields when present, and fall back to the legacy context field for old Gateway responses.
• Adds TypeScript and Python regression tests covering both new and legacy /recall response behavior.

Related Issue | 关联 Issue

None

Change Type | 修改类型

• Bug fix | Bug 修复
• New feature | 新功能
• Documentation update | 文档更新
• Code optimization | 代码优化

Self-test Checklist | 自测清单

• Verified locally | 本地验证通过
• No existing features affected | 无影响现有功能

Verified with:

• npm test
• npm run build:plugin
• python -m pytest hermes-plugin/memory/memory_tencentdb/tests -q
• git diff --check

Additional Notes | 其他说明

The legacy context field is preserved so existing clients can still read the full recall context. New clients can read appendSystemContext and prependContext separately.

[Maxwell-Code07]

Hi @lazyayuan,

We've received your PR regarding the missing prependContext in Gateway recall response. Thanks for catching this! We'll review and follow up.