[CICP-52129] Remove duplicate DELAY_REBALANCE_ENABLED from ResourceConfig

[sjainit]

Issues

• Addresses CICP-52129, under the Helix config rationalisation epic.

Description

Remove only the unused DELAY_REBALANCE_ENABLED copy from ResourceConfig, not delayed rebalancing itself.

• Delete ResourceConfigProperty.DELAY_REBALANCE_ENABLED and stop synthesizing it in mergeIdealStateWithResourceConfig.
• Make IdealState use its own enum constant and InstanceConfig use its existing enum constant.
• Preserve the serialized key, default true, getter/setter APIs, and active IdealState/InstanceConfig/ClusterConfig behavior.
• Preserve existing raw ResourceConfig fields as opaque metadata; do not delete or migrate persisted values.
• Update the directly related trimmer documentation and README compatibility guidance.

The ResourceConfig copy has no delayed-rebalance runtime reader in this repository. Fresh indexed searches for the removed qualified enum constant returned only linkedin/helix and the source-copy repository linkedin-sandbox/helixli; imported/static-import forms were also checked. No downstream MP reference requiring migration was found. This is indexed-source evidence, not a claim about all deployed binaries or live configurations.

Tests

• Local code review completed.
• Add default-value and legacy-record/true-false round-trip coverage in TestIdealState and TestInstanceConfig.
• Add ResourceConfig merge coverage for absent/true/false IdealState flags, null/existing ResourceConfig, preservation of arbitrary legacy metadata and unmodified inputs, and removal of the ResourceConfig enum member.
• Update existing constructor/merge assertions to verify the retired field is not synthesized.

JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk11.0.21-2-msft.jdk/Contents/Home \
  mvn -B -pl helix-core -am test \
  -Dtest=TestResourceConfig,TestIdealState,TestInstanceConfig \
  -Dsurefire.failIfNoSpecifiedTests=false -DfailIfNoTests=false

Result: 58 tests, 0 failures, 0 errors, 0 skips; BUILD SUCCESS. Core main sources compiled for the existing JDK 8 and JDK 11 targets, and core test sources compiled. Compiled-enum inspection confirms the ResourceConfig member is absent and the IdealState/InstanceConfig members remain. Full repository and integration suites were not run.

Changes that Break Backward Compatibility (Optional)

Direct source or compiled references to ResourceConfig.ResourceConfigProperty.DELAY_REBALANCE_ENABLED are incompatible with its removal. Use the API/enum for the actual IdealState, instance, or cluster scope and rebuild before upgrading Helix. Callers of the retained IdealState/InstanceConfig getter/setter APIs need no migration.

mergeIdealStateWithResourceConfig no longer adds this key when it is absent from ResourceConfig. A key already present in a raw ResourceConfig remains unchanged and has no delayed-rebalance effect. Do not automatically copy such a value into IdealState: that could activate a previously ignored setting.

No cluster-level or instance-level config is retired. No live configuration writes, deployments, or merges are part of this PR.

Documentation (Optional)

README configuration-compatibility section documents the retained settings, removed enum, raw-field behavior, and rebuild requirement.

Commits

One standalone commit references CICP-52129 and uses an imperative subject. The branch is based on current dev, not stacked on another config-removal PR.

Code Quality

Followed existing Helix formatting and TestNG/DataProvider conventions. git diff --check passes.

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