Fix SimplePublisher allowing re-subscription after cancellation

[zoewangg]

Motivation and Context

When a getObject ResponsePublisher is used as a putObject request body and the server returns a retryable error (e.g., 500), the SDK retries and re-subscribes to the same publisher chain. The SimplePublisher backing the ResponsePublisher (used by CRT HTTP client) had two bugs that caused the retry to hang:

1. subscribe() was missing a return after rejecting a duplicate subscriber, causing it to fall through and replace the subscriber reference.
2. On cancellation (triggered by ContentLengthAwareSubscriber.java#L60 after all bytes are consumed), subscriber was set to null, which allowed a new subscriber to be accepted into an exhausted stream with no data or terminal signal.

Modifications

• SimplePublisher.subscribe(): Added missing return after the duplicate subscriber error branch.
• SimplePublisher cancel handling: Removed subscriber = null on cancellation. The subscriber reference is kept so that re-subscription is properly rejected by the existing subscriber != null guard.
• AddingTrailingDataSubscriber.onSubscribe(): Propagate onError to downstream on duplicate subscription instead of silently swallowing it (defensive fix).

Testing

• SimplePublisherTest: Added subscribeAfterCancel_rejectsWithError and subscribeAfterCancelAndDataConsumed_rejectsWithError

• AddingTrailingDataSubscriberTest: Added duplicateOnSubscribe_shouldPropagateErrorToDownstream

• PutObjectRequestBodyFromSimplePublisherRetryTest: End-to-end test using WireMock — verifies putObject retry with a SimplePublisher-backed request body completes promptly instead of hanging

• Added unit tests

• Ran existing tests

• All new and existing tests passed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn clean install -pl :utils succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry

License

• I confirm that this pull request can be released under the Apache 2 license

[zoewangg]

Subscribe gets GC'd when the SimplePublisher itself is GC'd

[sonarqubecloud]

Quality Gate passed

Issues
6 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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