Adds capability to automatically switch to old access-control if model-group is excluded from protected resources setting

[DarshitChanpura]

Depends on: opensearch-project/security#5677

Description

Adds capability to automatically switch to old access-control if model-group is excluded from protected resources setting.

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[DarshitChanpura]

blocked by:
#4279

[DarshitChanpura]

Unable to reproduce the tests failures locally:

Tests with failures:
 - org.opensearch.ml.action.model_group.DeleteModelGroupTransportActionTests.test_ValidationFailedException
 - org.opensearch.ml.action.model_group.TransportUpdateModelGroupActionTests.test_UserSpecifiedRestrictedButNoBackendRolesField
 - org.opensearch.ml.action.model_group.TransportUpdateModelGroupActionTests.test_NoAccessUserUpdatingModelGroupException

[DarshitChanpura]

Found the root cause:
Since ResourceSharingClientAccessor is static and I added tests here that changes the value inside those tests it caused flaky-ness; as tests are run inside same jvm and if the last test that ran left the accessor assigned to some value, next test that didn't expect it would fail.

./gradlew :opensearch-ml-plugin:test                                                                                    
=======================================
OpenSearch Build Hamster says Hello!
  Gradle Version        : 8.14.3
  OS Info               : Mac OS X 15.7.1 (aarch64)
  JDK Version           : 21 (Eclipse Temurin JDK)
  JAVA_HOME             : /Users/dchanp/.sdkman/candidates/java/21.0.4-tem
  Random Testing Seed   : FE9A768CB2E8A43F
  Crypto Standard       : any-supported
=======================================

> Task :opensearch-ml-plugin:compileTestJava
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

...

BUILD SUCCESSFUL in 54s
27 actionable tasks: 2 executed, 25 up-to-date

[DarshitChanpura]

integTest failure:

Tests with failures:
 - org.opensearch.ml.rest.RestMLRAGSearchProcessorIT.testBM25WithCohere

not related to this PR.

Seems flaky as well since it passed on another run:
https://github.com/opensearch-project/ml-commons/actions/runs/18600818784/job/53038758361?pr=4244