Fix Reactor adapter async-context race in SentinelReactorSubscriber.currentContext

[wuwen5]

Describe what this PR does / why we need it

SentinelReactorSubscriber.currentContext() could throw NullPointerException: value under concurrent termination because it read currentEntry.getAsyncContext() twice. If AsyncEntry.exit() cleared async context between those reads, Context.put(...) received null.

• Race fix in context propagation

  • Updated currentContext() to use the already-captured local sentinelContext when putting Sentinel context into Reactor Context.
  • This removes the check-then-act race window created by re-reading currentEntry.getAsyncContext().

• Targeted regression coverage

  • Added SentinelReactorSubscriberTest in the reactor adapter test module.
  • The test drives a deterministic interleaving where currentContext() captures a non-null async context, a concurrent cancel path exits/clears context, and currentContext() must still propagate the captured value (the pre-fix path would pass null to Context.put).

com.alibaba.csp.sentinel.context.Context sentinelContext = currentEntry.getAsyncContext();
if (sentinelContext == null) {
    return actual.currentContext();
}
return actual.currentContext()
    .put(SentinelReactorConstants.SENTINEL_CONTEXT_KEY, sentinelContext);

26-05-21 09:28:59.436  WARN [x-server,,,TID:N/A] 766243 --- [ventExecutorLoop-1-4] [i.n.u.c.SingleThreadEventExecutor       :1002] [                 run] : Unexpected exception from an event executor:
java.lang.NullPointerException: value
        at java.util.Objects.requireNonNull(Objects.java:228)
        at reactor.util.context.Context5.put(Context5.java:61)
        at com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber.currentContext(SentinelReactorSubscriber.java:70)
        at com.alibaba.csp.sentinel.adapter.reactor.InheritableBaseSubscriber.onError(InheritableBaseSubscriber.java:169)
        at com.alibaba.csp.sentinel.adapter.reactor.SentinelReactorSubscriber.onError(SentinelReactorSubscriber.java:37)
        at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:544)
        at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onError(FluxOnAssembly.java:544)
        at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onError(FluxDoFinally.java:119)
        at reactor.core.publisher.FluxContextWrite$ContextWriteSubscriber.onError(FluxContextWrite.java:121)
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.onError(MonoFlatMap.java:172)
        at reactor.core.publisher.MonoFlatMap$FlatMapMain.secondError(MonoFlatMap.java:192)
        at reactor.core.publisher.MonoFlatMap$FlatMapInner.onError(MonoFlatMap.java:259)
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onError(MonoIgnoreThen.java:278)
        at reactor.core.publisher.FluxDoFinally$DoFinallySubscriber.onError(FluxDoFinally.java:119)
        at reactor.core.publisher.FluxPeek$PeekSubscriber.onError(FluxPeek.java:222)
        at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.onError(MonoIgnoreThen.java:278)
        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onError(MonoPeekTerminal.java:258)
        at org.springframework.http.server.reactive.ChannelSendOperator$WriteCompletionBarrier.onError(ChannelSendOperator.java:418)

某Redis Reactive请求先发生异常
→ Reactor进入onError链路
→ SentinelReactorSubscriber.currentContext()
→ Context.put(null)
→ NullPointerException
→ 未捕获异常打穿Netty EventExecutor
→ lettuce-eventExecutorLoop-1-4 号线程死亡(threadStatus=2)
→ executor terminated
→ 后续Redis Reactive callback无法调度
→ Spring Session Reactive Pipeline部分卡死 （重启恢复）

Does this pull request fix one issue?

fixed #3556 #1907

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