[4.x] Add `ServerModelReaderTest` from 3.x to MP OpenAPI unit tests

[tjquinno]

Environment Details

• Helidon Version: 4.x
• Helidon SE or Helidon MP
• JDK version:
• OS:
• Docker version (if applicable):

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Problem Description

As part of #7041 I realized a unit test we used to have in 3.x had been dropped moving to 4.x.

This issue tracks the need to bring the equivalent test forward into 4.x.

The 3.x test was in helidon/openapi which worked because the SE component depended on MP OpenAPI in 3.x. That is no longer true in 4.x, so the test will need to reside inhelidon/microprofile/openapi and be adapted to use the MP-friendly testing environment.