Fix Failing Clear Cache API Test

[naveentatikonda]

Description

Fix failing integration test. If there are any system indices in the cluster, the test will fail during validation as they are non-knn indices.

org.opensearch.knn.plugin.action.RestClearCacheHandlerIT > testClearCacheMultipleIndices FAILED
    org.opensearch.client.ResponseException: method [POST], host [https://127.0.0.1:52190/], URI [/_plugins/_knn/clear_cache/_all], status line [HTTP/2.0 500 Internal Server Error]
    {"error":{"root_cause":[{"type":"k_n_n_invalid_indices_exception","reason":"ClearCache request rejected. One or more indices have 'index.knn' set to false."}],"type":"k_n_n_invalid_indices_exception","reason":"ClearCache request rejected. One or more indices have 'index.knn' set to false."},"status":500}

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5e2f899) 85.13% compared to head (95e8603) 85.13%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #1336   +/-   ##
=========================================
  Coverage     85.13%   85.13%           
  Complexity     1210     1210           
=========================================
  Files           160      160           
  Lines          4931     4931           
  Branches        449      449           
=========================================
  Hits           4198     4198           
  Misses          537      537           
  Partials        196      196           

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[vamshin]

LGTM! Thanks