NullPointerException in McpClientSession

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Bug description

In https://github.com/enola-dev/enola/issues/1862, I've run into:

java.lang.NullPointerException: data
	at java.base/java.util.Objects.requireNonNull(Objects.java:259)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.next(FluxHandle.java:251)
	at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$12(McpClientSession.java:280)
	at reactor.core.publisher.FluxHandle$HandleSubscriber.onNext(FluxHandle.java:113)
	at reactor.core.publisher.SerializedSubscriber.onNext(SerializedSubscriber.java:99)
	at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.onNext(FluxTimeout.java:181)
	at reactor.core.publisher.MonoCreate$DefaultMonoSink.success(MonoCreate.java:176)

I guess that this means that this is null?

Environment

0.14.1

Steps to reproduce

Unfortunately I'm unable to reproduce it locally even myself, but I am consistently seeing this on a failing CI build.

Expected behavior

Clear error message instead of NPE.

Minimal Complete Reproducible example

N/A

Comienza en la línea 280 de mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java y revisa el flujo de sendRequest en torno a la NPE de FluxHandle reportada. Usa como contexto el fallo descrito en enola-dev/enola#1862 y el entorno 0.14.1; se considera terminado cuando el fallo ya no produce una NPE poco informativa, sino que informa de un error claro.