Fix invalid message in OpenAPI gen client

[NipunaMarcus]

Purpose

This will fix the openapi gen client message issue in OpenAPI tooling.

Fixes #20384

Check List

• Read the Contributing Guide
• Updated Change Log
• Checked Tooling Support (#)
• Added necessary tests
  • Unit Tests
  • Spec Conformance Tests
  • Integration Tests
  • Ballerina By Example Tests
• Increased Test Coverage
• Added necessary documentation
  • API documentation
  • Module documentation in Module.md files
  • Ballerina By Examples

[codecov-io]

Codecov Report

Merging #20387 into release-stage will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release-stage   #20387   +/-   ##
==============================================
  Coverage          15.21%   15.21%           
==============================================
  Files                 50       50           
  Lines               1341     1341           
  Branches             208      208           
==============================================
  Hits                 204      204           
  Misses              1121     1121           
  Partials              16       16

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