PenTest fixes: error codes and update model group fix

[rbhavna]

Description

PenTest fixes: error codes and update model group fix

Issues Resolved

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Merging #1074 (9089f31) into 2.x (abdccc0) will decrease coverage by 0.07%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##                2.x    #1074      +/-   ##
============================================
- Coverage     77.98%   77.91%   -0.07%     
+ Complexity     2064     2060       -4     
============================================
  Files           167      167              
  Lines          8534     8535       +1     
  Branches        848      848              
============================================
- Hits           6655     6650       -5     
- Misses         1502     1507       +5     
- Partials        377      378       +1     

Flag	Coverage Δ
ml-commons	77.91% <81.81%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...org/opensearch/ml/rest/RestMLPredictionAction.java	48.71% <0.00%> (ø)
...n/model_group/TransportUpdateModelGroupAction.java	77.86% <80.00%> (ø)
...arch/ml/action/models/GetModelTransportAction.java	82.35% <100.00%> (+0.35%)	⬆️
...search/ml/action/tasks/GetTaskTransportAction.java	62.50% <100.00%> (ø)
...n/java/org/opensearch/ml/model/MLModelManager.java	62.63% <100.00%> (ø)
...ensearch/ml/rest/RestMLUpdateModelGroupAction.java	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes