mcp: streamable transport terminating stream prematurely

[aryannr97]

There's a race condition between the notification being queued and the tool response clearing streamRequests. The response can trigger premature stream termination before notifications are sent.

To Reproduce

$ go test -timeout 60s -run ^TestStreamableServerTransport$ github.com/modelcontextprotocol/go-sdk/mcp -race 
--- FAIL: TestStreamableServerTransport (0.02s)
    --- FAIL: TestStreamableServerTransport/tool_notification (0.00s)
        streamable_test.go:556: received unexpected messages (-want +got):
              []jsonrpc2.Message{
                &jsonrpc2.Request{Method: "notifications/progress", Params: `{"progress":0,"progressToken":null}`},
            -   &jsonrpc2.Response{Result: json.RawMessage(`{"content":null}`), ID: jsonrpc2.ID{value: int64(2)}},
              }
FAIL
FAIL    github.com/modelcontextprotocol/go-sdk/mcp      0.633s
FAIL

Expected behavior
Response stream should not terminate prematurely and messages should not be lost