fix: pass connected client to proxy to reuse session

[wzxxing]

https://gofastmcp.com/servers/proxy#session-isolation-&-concurrency

Summary

Changes

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The client passed to the proxy needs to be already connected, otherwise the proxy will create a new session to connection for every request

reference: https://gofastmcp.com/servers/proxy#session-isolation-&-concurrency

Fresh Sessions

When you pass a disconnected client (which is the normal case), each request gets its own isolated backend session:

User experience

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