Add endpoint for rotating multiple index sets

[moesterheld]

Description

Adds bulk action for rotating mulitple index sets.

Motivation and Context

needed for incompatible indices handling on OpenSearch upgrade page

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have requested a documentation update.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[Copilot]

Pull request overview

Adds a bulk deflector cycling endpoint to rotate multiple index sets in one request, primarily to support upgrade flows (e.g., OpenSearch incompatible indices handling) while reusing the existing bulk-operation infrastructure.

Changes:

• Add POST /system/deflector/bulk_cycle endpoint executing per-index-set cycling with aggregated BulkOperationResponse.
• Refactor single-index-set cycle to share the same inner execution path used by the bulk executor.
• Add cluster proxy (/cluster/deflector/bulk_cycle) and a Retrofit remote interface method, plus a dedicated unit test for the new bulk behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
graylog2-server/src/test/java/org/graylog2/rest/resources/system/DeflectorResourceTest.java	Adds unit tests covering success/failure aggregation, empty request rejection, and permission failures for bulkCycle.
graylog2-server/src/main/java/org/graylog2/shared/rest/resources/system/RemoteDeflectorResource.java	Introduces Retrofit client method for the new bulk-cycle endpoint (needs Retrofit @Body annotation fix).
graylog2-server/src/main/java/org/graylog2/rest/resources/system/DeflectorResource.java	Adds the bulk-cycle REST endpoint backed by SequentialBulkExecutor and shared cycleInner.
graylog2-server/src/main/java/org/graylog2/rest/resources/cluster/ClusterDeflectorResource.java	Adds cluster-level proxy endpoint for bulk cycling (currently does not return the bulk result).

Comments suppressed due to low confidence (1)

graylog2-server/src/main/java/org/graylog2/rest/resources/cluster/ClusterDeflectorResource.java:81

• Method name bulkcycle violates Java lowerCamelCase conventions and is inconsistent with cycle(...)/cycleIndexSet(...) in the same class. Rename to bulkCycle for readability (endpoint path is unaffected).

    public void bulkcycle(@Parameter(name = "Entities to cycle", required = true) final BulkOperationRequest bulkOperationRequest) throws IOException {

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