[FLINK-38770][runtime] Fix parallelism overrides ignored in Application Mode

[barakbaron12]

What is the purpose of the change

This PR fixes a bug where pipeline.jobvertex-parallelism-overrides configuration was completely ignored when running jobs in Application Mode with Flink 2.0+. This broke the Flink Kubernetes Operator's autoscaler functionality.

Brief change log

• Created ParallelismOverrideUtil to centralize override application logic
• Modified all scheduler factories (DefaultScheduler, AdaptiveScheduler, AdaptiveBatchScheduler) to apply overrides after StreamGraph→JobGraph conversion
• Removed duplicate override logic from Dispatcher.java
• Ensured correct configuration precedence: job config > job master config

Verifying this change

This change added tests and can be verified as follows:

• Added 7 unit tests in ParallelismOverrideUtilTest covering all override scenarios
• Added 4 integration tests in ParallelismOverridesITCase verifying end-to-end behavior across different scheduler types
• Modified existing integration tests in AdaptiveBatchSchedulerITCase and AdaptiveSchedulerITCase to include parallelism override tests

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: yes (JobManager scheduler factories)
• 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:

• 07aa475253453481c4818297dc9fccf068ea689c Azure: FAILURE

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