mcp: close HTTP response body from DELETE request in streamableClientConn.Close

[blackwell-systems]

Summary

The DELETE request that terminates a streamable HTTP session discards the *http.Response without closing its body. Per the net/http documentation, this prevents the underlying TCP connection from being reused and leaks it.

The bug

// mcp/streamable.go:2239 (before)
} else if _, err := c.client.Do(req); err != nil {
    c.closeErr = err
}

On the success path (no error), the response is silently discarded. The default client is http.DefaultClient with keep-alives enabled, so the leaked connection stays open until GC or process exit.

The fix

// mcp/streamable.go:2239 (after)
} else if resp, err := c.client.Do(req); err != nil {
    c.closeErr = err
} else {
    resp.Body.Close()
}

All 10 other client.Do call sites in this file close the response body. This one was missed.

Test plan

• go build ./... passes
• go test ./... -short passes

Fixes #928