Add benefit demonstration test for FlowDex token budgeting

[aandersen2323]

Summary

• add an integration-style pytest that exercises run_inference with constrained budgets
• stub Redis and LLM calls to automatically compare FlowDex token usage with a naive baseline

Testing

• pytest server/tests/test_benefit_demo.py
• pytest server/tests

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https://chatgpt.com/codex/tasks/task_e_68fafe4ecc6c83319a0ecb538ada75d7