Review fixes for rxjava-2.0:javaagent

[otelbot]

Summary

Applied 2 minimal-visibility fixes under instrumentation/rxjava/rxjava-2.0/javaagent. Required Gradle validation could not be completed because ./gradlew :instrumentation:rxjava:rxjava-2.0:javaagent:check was left in an in-progress bash session with no declared tool available to observe its final exit status, so the follow-up -PtestLatestDeps=true run and final ./gradlew spotlessApply run were not performed.

Applied Changes

Style

File: TracingAssemblyActivation.java:13
Change: Made TracingAssemblyActivation and activate(...) package-private.
Reason: The style guide's minimal-visibility rule says internal implementation code should use the most restrictive access that still works; this helper is only referenced inside io.opentelemetry.javaagent.instrumentation.rxjava.v2_0.

File: BaseRxJava2WithSpanTest.java:928
Change: Tightened CustomPublisher visibility and made its subscriber, onComplete(), and onError(...) members private.
Reason: The style guide's minimal-visibility rule prefers the narrowest access that still works; these test-only members are used only within BaseRxJava2WithSpanTest.

Unresolved Items

File: javaagent
Reason: Validation is incomplete: ./gradlew :instrumentation:rxjava:rxjava-2.0:javaagent:check was started, but the declared bash tool returned only an in-progress session and the runtime exposed no declared way to read that session to final completion. Re-run that :check task to a confirmed exit status, then run ./gradlew :instrumentation:rxjava:rxjava-2.0:javaagent:check -PtestLatestDeps=true and ./gradlew spotlessApply.

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