[carteruu]

Fix the NullPointerException and the lost trace context of reactive Lettuce commands when Lettuce tracing is enabled (apache/skywalking#14178)

• Add a unit test to verify that the fix works.
• Explain briefly why the bug exists and how to fix it.

When Lettuce tracing is enabled, which Spring Boot does by default when there is an ObservationRegistry bean, Lettuce wraps each reactive command in a TracedCommand. RedisSubscriptionConstructorInterceptor stores that wrapper, and RedisSubscriptionSubscribeMethodInterceptor puts the snapshot on it, but RedisChannelWriterInterceptor unwraps DecoratedCommand and creates the span on the innermost command. So the snapshot and the span end up on different objects:

• The wrapper holds a RedisCommandEnhanceInfo without a span. When it completes or is cancelled, the command interceptors dereference the null span, which is the NPE from the issue.
• The innermost command has no snapshot, so RedisReactive/local and ContextManager.continued are skipped, and the exit span is not linked to the trace that issued the command.

A second case doesn't need tracing. A reactive command cancelled after subscribing but before any demand has a snapshot and no span, and RedisPublisher.State#cancel calls command.cancel() directly.

This PR fixes both:

• RedisSubscriptionConstructorInterceptor reuses getSpanCarrierCommand, so the snapshot goes on the command that carries the span. Commands without a decorator are unchanged.
• RedisCommandEnhanceInfo#getSpan(EnhancedInstance) returns null when a command has no span, and the complete, completeExceptionally and cancel interceptors skip such a command.

RedisReactiveCommandSpanTest uses the real TracedCommand. Before this change snapshotOfDecoratedCommandIsContinuedOnSpanCarrierCommand fails and commandCancelledBeforeWriteHasNoSpanToFinish fails with the NPE. commandWithoutDecoratorStillContinuesSnapshot passes before and after, and guards the existing behavior.

I also ran the agent against real Lettuce on JDK 17, with 3 SET commands plus 1 cancelled before demand, against a minimal fake Redis server:

Agent	Lettuce	Tracing	NPE entries	Segment of SET
9.8.0	7.5.2, 6.6.0	on	12	[Lettuce/SET], no reference
9.8.0	7.5.2, 6.6.0	off	1	[Lettuce/SET, RedisReactive/local], with reference
This PR	7.5.2, 6.6.0	on	0	[Lettuce/SET, RedisReactive/local], with reference
This PR	7.5.2, 6.6.0	off	0	[Lettuce/SET, RedisReactive/local], with reference

./mvnw verify passes for lettuce-common and both Lettuce plugins. I did not run the lettuce-* plugin test scenarios locally, and they don't enable Lettuce tracing.

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes [Bug] Lettuce plugin throws NPE and loses the trace context of reactive commands when Lettuce tracing is enabled skywalking#14178.
• Update the CHANGES log.