mcp: Client.Connect succeeds after subscriptions/listen is rejected

When a v2026-07-28 streamable HTTP client has notification handlers installed, Client.Connect opens subscriptions/listen after server/discover. If the server immediately rejects that subscriptions/listen request with a JSON-RPC error, Connect currently still returns a usable session.

What I did:

I added a regression test with a fake streamable server that returns a successful server/discover response, then returns a JSON-RPC InvalidParams error for subscriptions/listen. The client is configured with ToolListChangedHandler so it attempts the listen call during connect.

What I saw:

On current origin/main at 64e454e35c23, the regression test fails because Connect succeeds even though the listen request was rejected.

What I expected:

Connect should fail and surface the subscriptions/listen error. Otherwise callers believe notification subscriptions are active when the server already rejected them.

Version:

origin/main at 64e454e35c23

Go version:

go version go1.26.5 darwin/arm64

Start at the Go Client.Connect entry point and the regression test described in the issue, then trace the server/discover and subscriptions/listen requests when ToolListChangedHandler is configured. Run the fake streamable server test and verify that a rejected subscriptions/listen JSON-RPC request makes Connect return the surfaced error rather than a usable session.