fix(frontend): proxy LangGraph SSE without buffering

[LittleChenLiya]

Summary

• 将 /api/langgraph/* 从 Next rewrite 改为 App Router Route Handler 代理，避免 SSE 响应被 rewrite 路径缓冲。
• 代理时保留 upstream response body 的流式返回，并设置 Cache-Control: no-cache, no-transform 与 X-Accel-Buffering: no。
• 将非 LangGraph 的 gateway API rewrite 改为显式列表，避免 catch-all /api/:path* 抢走 /api/langgraph/*。

背景

本地复现时，直接访问后端 127.0.0.1:8001/api/threads/.../runs/stream 可以增量收到 SSE；但从前端同源路径 localhost:3000/api/langgraph/.../runs/stream 访问时，metadata、messages 和 end 会在 run 结束后一起到达，导致 UI 表现为等待较久后一次性刷新。

这个问题不是单纯缺少 messages-tuple stream mode；即使请求 messages-tuple，rewrite 路径仍会缓冲 SSE。

[Copilot]

Pull request overview

This PR changes how the frontend proxies LangGraph-compatible gateway endpoints by replacing Next.js rewrites with an App Router Route Handler, aiming to preserve SSE streaming behavior (avoid response buffering) on /api/langgraph/*.

Changes:

• Add /api/langgraph/[[...path]] Route Handler that proxies to the gateway while streaming the upstream response body and normalizing headers to discourage buffering/transforms.
• Add unit tests to validate URL construction, header forwarding/stripping, and streaming response behavior.
• Replace the previous /api/:path* catch-all gateway rewrite with an explicit allowlist to prevent it from intercepting /api/langgraph/*.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
frontend/src/app/api/langgraph/[[...path]]/route.ts	Introduces a Node.js runtime Route Handler proxy for LangGraph paths with streaming response passthrough and header normalization.
frontend/tests/unit/app/api/langgraph-route.test.ts	Adds unit coverage for URL encoding, header filtering, request body streaming, and response header normalization.
frontend/next.config.js	Removes /api/langgraph rewrites and replaces the gateway catch-all rewrite with an explicit set of gateway route rewrites.