Fix bug where admin can read system index

[derek-ho]

Description

Prevents reads on security system index registered through core

Issues Resolved

Fix: #4755
Is this a backport? If so, please add backport PR # and/or commits #, and remove backport-failed label from the original PR.
No
Do these changes introduce new permission(s) to be displayed in the static dropdown on the front-end? If so, please open a draft PR in the security dashboards plugin and link the draft PR here
No

Testing

Added unit test

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.
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[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.96%. Comparing base (830b341) to head (7459bc2).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rity/configuration/SystemIndexSearcherWrapper.java	33.33%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4774      +/-   ##
==========================================
- Coverage   67.98%   67.96%   -0.02%     
==========================================
  Files         310      310              
  Lines       20924    20926       +2     
  Branches     3318     3321       +3     
==========================================
- Hits        14225    14223       -2     
+ Misses       4951     4950       -1     
- Partials     1748     1753       +5     

Files with missing lines	Coverage Δ
...figuration/SecurityFlsDlsIndexSearcherWrapper.java	91.66% <ø> (ø)
...rity/configuration/SystemIndexSearcherWrapper.java	91.37% <33.33%> (ø)

... and 5 files with indirect coverage changes