Ensure that plugin can update on system index when utilizing pluginSubject.runAs

[cwperks]

Description

Test run in JS w/ failure related to update request: https://github.com/opensearch-project/job-scheduler/actions/runs/12934907118/job/36096320325?pr=714

Similar to #5032, but for the use-case where a plugin issues an update request.

While testing opensearch-project/job-scheduler#714, I ran into an issue where a plugin was not able to perform update operations on an index when replacing try (ThreadContext.StoredContext ctx = threadContext.stashContext()) { ... } with the new replacement pluginSubject.runAs(() -> { ... }) because the security plugin calls dlsFlsValve.invoke after privilege evaluation is complete and successful for regular users. This PR updates the codepaths to account for the introduction of pluginSubject as a separate form of a system-level request.

• Category (Enhancement, New feature, Bug fix, Test fix, Refactoring, Maintenance, Documentation)

Bug fix

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.51%. Comparing base (472f7c7) to head (5a103eb).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5055   +/-   ##
=======================================
  Coverage   71.51%   71.51%           
=======================================
  Files         336      336           
  Lines       22621    22623    +2     
  Branches     3600     3600           
=======================================
+ Hits        16177    16179    +2     
+ Misses       4645     4644    -1     
- Partials     1799     1800    +1     

Files with missing lines	Coverage Δ
...search/security/configuration/DlsFlsValveImpl.java	64.28% <100.00%> (+0.24%)	⬆️

... and 8 files with indirect coverage changes

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/security/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/security/backport-2.x
# Create a new branch
git switch --create backport/backport-5055-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 ec99e7eb0191e521b2c7046ba3fbfcc633cac6fc
# Push it to GitHub
git push --set-upstream origin backport/backport-5055-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/security/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-5055-to-2.x.