Increase Test Coverage for Open AI Client Builder

[v-lrdev]

This PR adds unit tests for the OpenAIClientBuilder class to verify the correct behavior of its methods, including:

Building clients with different credentials and configurations.
Setting and verifying various options and policies.
Ensuring type-safe handling of HttpPipelinePolicy lists.

Changes

• Added unit tests for OpenAIClientBuilder methods.
• Used reflection to access and verify private fields.
• Ensured type-safe handling of HttpPipelinePolicy lists.

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[azure-sdk]

API change check

API changes are not detected in this pull request.