Refactor prompt_eval.py to remove repetitive code

[tectonia]

Remove repetitive common parameters and if/else handling of flow types to improve readability
Example:
https://github.com/microsoft/genaiops-promptflow-template/blob/a22290d1cdcd810ef5f4d78739851322b719233d/llmops/common/prompt_eval.py#L237C1-L273C62

• Add unit tests after refactoring