feat(adapters): add LangChain memory middleware

[March-77]

Description | 描述

Adds a focused LangChain v1 / LangGraph middleware adapter on top of the shared Gateway adapter kit from #316.

• maps beforeAgent to Gateway /recall
• maps afterAgent to Gateway /capture
• resolves a stable LangGraph thread/session identity through host runtime context
• normalizes LangChain BaseMessage values to JSON-safe { role, content } messages
• fails open by default when the Gateway is unavailable, with an opt-in fail-closed mode
• adds focused unit tests and a Mermaid data-flow/usage guide
• does not add LangChain as a core dependency; the host injects its installed createMiddleware

Related Issue | 关联 Issue

Related to #235
Depends on #316

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

Verification:

• npm.cmd exec vitest run src/adapters/langchain/langchain.test.ts src/adapters/gateway-client/gateway-client.test.ts: 8 passed
• npm.cmd test: 75 passed
• npm.cmd run build:plugin: passed
• git diff --check: passed

Additional Notes | 其他说明

This is intentionally a small platform-specific follow-up. It reuses #316 instead of introducing another Gateway client or adapter SDK. The PR is opened as draft while #316 is pending; once #316 lands, this branch can be rebased so the review diff contains only the LangChain middleware commit.

[Maxwell-Code07]

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