chore(opensearch): improve error message generation logic

[mazyu36]

Issue # (if applicable)

N/A

Reason for this change

Modified the code so that error messages don't need to be changed when adding new instance types in #32592.

Ref: #32592 (comment)

In #32592 , refactoring was only done for bug-related parts, so this PR will refactor the remaining areas.

Description of changes

• Improve error message generation logic for encryption at rest and ultra warm.
• Refactor unit tests.

Describe any new or updated permissions being added

Nothing

<!— What new or updated IAM permissions are needed to support the changes being introduced ? -->

Description of how you validated changes

Re-run unit tests and integ tests

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.54%. Comparing base (dcf1d74) to head (c6a5b1d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #32651   +/-   ##
=======================================
  Coverage   80.54%   80.54%           
=======================================
  Files         106      106           
  Lines        6954     6954           
  Branches     1287     1287           
=======================================
  Hits         5601     5601           
  Misses       1175     1175           
  Partials      178      178           

Flag	Coverage Δ
suite.unit	80.54% <ø> (ø)

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Components	Coverage Δ
packages/aws-cdk	80.54% <ø> (ø)

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Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).

[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: c6a5b1d
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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