[FLINK-39311][runtime] Fix application-related main thread execution …

[eemario]

…in Dispatcher

What is the purpose of the change

This pull request fixes the main thread execution violation in the Dispatcher and improves the associated tests.

Brief change log

• Use thenComposeAsync instead of thenCompose to enforce main thread execution for removeApplication()

Verifying this change

Please make sure both new and modified tests in this PR follow the conventions for tests defined in our code quality guide.

This change is already covered by existing tests, such as DispatcherApplicationTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[flinkbot]

CI report:

• 70b4d91 Azure: SUCCESS

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