[FLINK-39963][runtime/tests] Stabilize balanced input slow task detector test

[qiuyanjun888]

What is the purpose of the change

This pull request stabilizes ExecutionTimeBasedSlowTaskDetectorTest.testBalancedInput by making the execution-time relationship used by the assertion explicit instead of relying on wall-clock timing between task deployment and completion.

Brief change log

• Sets deterministic DEPLOYING / FINISHED timestamps for the running and finished executions in testBalancedInput.
• Keeps the test focused on the balanced-input slow-task detection expectation: two still-running tasks should exceed the finished baseline.

Verifying this change

This change is already covered by existing tests, such as:

• ./mvnw -pl flink-runtime -Dtest=ExecutionTimeBasedSlowTaskDetectorTest#testBalancedInput test
• ./mvnw -pl flink-runtime -Dtest=ExecutionTimeBasedSlowTaskDetectorTest test

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

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• Yes (please specify the tool below)

Generated-by: OpenAI GPT-5.5 via Hermes Agent

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CI report:

• 5f9af79 Azure: SUCCESS

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