[FLINK-40143][core] Fix displayed restart strategy for config option based restart strategies

[csurong]

What is the purpose of the change

Fixes FLINK-40143 on the release-1.20 branch.

Jobs configured through the restart-strategy.* options use the correct job-level restart strategy at runtime, but the archived execution configuration exposed by /jobs/:jobid/config and the Dashboard can display Cluster level default restart strategy.

Current master is not affected because FLINK-36249 changed ArchivedExecutionConfig to derive this description from the configuration. This PR is a targeted compatibility fix for release-1.20 that also preserves the behavior of the deprecated explicit restart strategy APIs.

Brief change log

• Derive the archived restart strategy description from ExecutionConfig.toConfiguration() when the deprecated restart strategy object is the fallback strategy.
• Preserve explicitly configured deprecated restart strategy objects as the higher-priority source.
• Add regression coverage for fixed-delay, no-restart, failure-rate, exponential-delay, fallback/default, and legacy execution retry settings.

Verifying this change

This change added tests and can be verified as follows:

• mvn -q -Denforcer.skip -pl flink-core -Dtest=ExecutionConfigTest test
• mvn -q -Denforcer.skip -pl flink-runtime -Dtest=JobConfigHandlerTest,RestartBackoffTimeStrategyFactoryLoaderTest test

Both commands pass on the latest release-1.20 branch.

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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Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex)

Generated-by: OpenAI Codex (GPT-5)

[flinkbot]

CI report:

• 53a5d5c Azure: SUCCESS

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