[FLINK-36973][config] Skip options not explicitly set when converting SavepointRestoreSettings to Configuration

[AlexYinHan]

What is the purpose of the change

This PR fixes the issue in FLINK-39673. It skips writing RESTORE_MODE and SAVEPOINT_IGNORE_UNCLAIMED_STATE from SavepointRestoreSettings into Configuration when they were not explicitly provided. If the user uses deprecated configuration keys for these options, this prevents the configurations from being overridden by default values.

Brief change log

• In SavepointRestoreSettings, change the allowNonRestoredState and recoveryClaimMode from NonNull to Nullable, to tell whether they are explicitly provided.
• Modify SavepointRestoreSettings.toConfiguration() to skip writing RESTORE_MODE and
  SAVEPOINT_IGNORE_UNCLAIMED_STATE when they were not explicitly provided by the user.

Verifying this change

This change added tests and can be verified as follows:

• SavepointRestoreSettingsTest.java Added test that validates that default/null values of SavepointRestoreSettings are correctly handled

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)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

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Generated-by: [Claude-4.6-Opus]

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CI report:

• 6781205 Azure: FAILURE

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