Fix to call the duplicator's child stream subscriber methods by the correct executor

[minwoox]

Motivation: Subscriber methods must be called by the executor that is specified when subscribing to a StreamMessage. However, the methods of the duplicator's child stream subscriber are currently being called from the duplicator's executor, which is incorrect.

Modifications:

• Updated the implementation to ensure that the duplicator's child stream subscriber methods are called by the correct executor as specified during subscription.

Result:

• The duplicator's child stream subscriber methods are now called by the correct executor.

[github-actions[bot]]

🔍 Build Scan® (commit: 2d7fd3671c76b39a63478330169db32ad9e8b663)

Job name	Status	Build Scan®
build-windows-latest-jdk-21	✅	https://ge.armeria.dev/s/sgelch2c6grp4
build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/wpsz4e527sxae
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/d2xb5wnax3qn6
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/ngnpcqu3qe5os
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/frf5w6sqxixq6
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/hasiyzm6t6dxg
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/fm2z4qwwei7au
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/ge2okoorkgczw

[minwoox]

Static analysis was not easy because two executors were used together in StreamMessageProcessor. How about refactoring the implementation later to delegate the logic to DownstreamSubscription if DownstreamSubscription.executor is needed?

That's a good suggestion. 👍 Addressed it from this PR. :wink:

[minwoox]

Thanks for the quick review. 🙇 I'll test it with the snapshot version to ensure the problem is fixed.